The world of enterprise communication has evolved significantly over the past several years. Traditional phone systems that once defined the enterprise landscape are giving way to cloud-first architectures that prioritize agility, scalability, and user-centric experiences. At the center of this transformation is the shift from legacy PBX systems to cloud-based collaboration platforms that integrate calling, messaging, meetings, and more into a single unified solution.
Cisco has been a key player in enabling this transition with its offering of Webex Teams Calling. Designed for businesses seeking to modernize their telephony environment without compromising existing investments, Webex Teams Calling allows for seamless migration to the cloud at a pace dictated by the customer. This is especially beneficial to organizations that are still reliant on their on-premises PSTN infrastructure or have long-standing contracts with telephony carriers.
Webex Teams Calling was built to address a critical gap in the market. While previous Cisco solutions, such as Cisco Spark Call, offered a fully cloud-based calling platform including PSTN access, it primarily suited customers who were ready to abandon their on-prem systems entirely. Many businesses, however, were not in a position to make such a decisive shift. Some were tied into long-term PSTN contracts. Others preferred a hybrid approach, where only parts of their organization would migrate to the cloud initially. These customers required a solution that could maintain a unified dial plan and calling experience across both cloud and on-premise users. Webex Teams Calling was developed precisely for that need.
This solution enables organizations to move their entire user base or just a selected subset to a modern, cloud-first PBX while maintaining the use of their existing PSTN circuits. Whether they use traditional TDM-based PRI lines, SIP trunks, or even legacy PBXs for PSTN connectivity, customers can now integrate those resources directly with the Webex Collaboration Cloud through a centralized architecture built around Cisco’s Unified Border Element, commonly referred to as the CUBE.
The Value of Gradual Migration
One of the greatest strengths of Webex Teams Calling is its flexibility. Rather than forcing a full-scale, immediate migration, businesses can move to the cloud in stages. This model supports cloud-only deployments, fully on-premise systems, or any combination of the two, based on business priorities and technical feasibility.
This flexibility is supported through Cisco’s Collaboration Flex Plan. This licensing model brings together calling, messaging, and meeting capabilities into a single, simple subscription. It not only makes licensing easier to manage but also provides the foundation for transitioning users between different collaboration modes without disrupting service or incurring additional overhead.
Webex Teams Calling supports both centralized and distributed calling architectures. It integrates seamlessly with existing Cisco infrastructure and endpoints, including the popular 6800 and 8800 series desk phones, allowing businesses to preserve their current hardware investments. The ability to maintain endpoint continuity is a key advantage, especially for large enterprises with hundreds or thousands of deployed devices.
From an operational perspective, the inclusion of Webex Teams as the collaboration interface enhances communication beyond voice. Users gain access to persistent messaging, file sharing, and integrated meetings, all from the same platform that handles their business calling. This unified approach streamlines workflows and improves productivity across the organization.
Retaining Existing PSTN Connectivity
The critical enabler in this hybrid calling model is the ability to reuse existing PSTN connections. Whether using a digital PRI line, a SIP trunk to a modern IP carrier, or a connection to a legacy PBX, Webex Teams Calling supports seamless integration with existing telephony infrastructure. This capability is enabled through the SIP trunk that links the Webex Collaboration Cloud to the on-premises CUBE device.
The SIP trunk serves as the bridge between cloud-registered endpoints and local PSTN infrastructure. When a user places a call from a Webex Teams-connected device, the Webex dial plan evaluates the dialed number. If the number does not match any internal extensions or registered directory numbers, the call is forwarded down the SIP trunk to the CUBE.
The CUBE then evaluates the call using its dial-peer configurations to determine the appropriate routing path. This may result in the call being sent directly to a PRI circuit, to a SIP trunk connected to an external IP-PSTN provider, or to a local PBX that further processes the call. This approach gives IT teams maximum control over call routing and ensures that users have uninterrupted access to PSTN services during and after the migration process.
This model also supports centralized call routing for geographically dispersed organizations. Calls from multiple locations can be routed through a single CUBE and PSTN gateway, or separate CUBEs can be deployed at each site to ensure local call breakout and minimize toll charges. The architecture can be customized to align with corporate dial plans, regulatory requirements, and cost optimization strategies.
Overview of PSTN Connectivity Options
Webex Teams Calling provides three primary options for connecting to PSTN services using an on-premise CUBE. Each of these options leverages the SIP trunk between the cloud and the local CUBE to route calls that are not handled by the Webex internal dial plan.
The first option involves routing calls from the Webex Cloud to the CUBE and then out through a traditional PRI circuit. This is ideal for businesses that have existing digital lines and wish to preserve that investment while adopting cloud collaboration tools.
The second option sends the call from the CUBE to an IP-based PSTN provider over a SIP trunk. This method provides more scalability and is generally more flexible and cost-effective than TDM-based solutions.
The third option involves routing the call from the CUBE to an on-prem PBX, which then either completes the call internally or routes it to the PSTN. This is useful in environments where a legacy PBX still plays a significant role in call processing, and organizations are not ready to fully retire that infrastructure.
In all three scenarios, the Webex Teams user interface remains consistent. From the end-user perspective, calling behaviors do not change, even as the backend architecture is adapted to meet operational or financial requirements. This transparency is essential to maintaining productivity and minimizing support overhead during the migration.
Each of these PSTN connectivity options will be explored in detail in the next parts of this guide. By understanding how each model works, businesses can make informed decisions about how to design their hybrid cloud telephony environment to support current needs and future growth.
PSTN Connectivity Option 1: Webex to CUBE to PRI
In the context of Webex Teams Calling, integrating existing telephony infrastructure with modern cloud collaboration tools is essential for many organizations seeking a balanced transition strategy. One of the most straightforward methods of bridging cloud and legacy systems is by connecting the Webex Collaboration Cloud to a traditional PSTN circuit through Cisco Unified Border Element (CUBE) and a Primary Rate Interface (PRI) connection. This model, commonly known as Webex to CUBE to PRI, provides a reliable foundation for businesses that want to preserve their current TDM-based telephony investments while adopting cloud-first communication strategies.
This deployment scenario begins with a call being placed by a user on a device registered to the Webex Collaboration Cloud. These endpoints could be Cisco desk phones, software clients, or any hardware that supports registration to the Webex platform. When a user dials a number, the system first checks the Webex cloud’s dial plan to see if it is an internal call or corresponds to a registered extension. If there is no match, indicating that the number is external, the system routes the call via SIP trunk to the on-premises CUBE.
CUBE serves as the critical interface between the cloud and the organization’s on-site voice infrastructure. Once the call reaches the CUBE, it evaluates the dialed digits using configured dial-peers. These dial-peers are routing rules that determine how the call should be processed based on the number pattern. In the Webex to CUBE to PRI model, the matching dial-peer will route the call to the digital PRI line that connects the enterprise to the PSTN.
A PRI line provides 23 bearer channels for simultaneous voice calls and one signaling channel. Despite being considered a legacy technology by some, PRI circuits remain widely deployed across many industries and regions. They are often favored for their reliability, quality of service, and compliance with local telephony regulations. Enterprises that have built critical services around PRI connectivity can continue to utilize these investments while introducing cloud-based features through Webex.
From an infrastructure perspective, this model requires no changes to the external PSTN-facing architecture. Existing PRI gateways and circuits remain in place. The only required addition is the integration of CUBE to serve as the intermediary for cloud-initiated calls. CUBE ensures seamless signaling conversion between SIP-based cloud protocols and the TDM signaling used over PRI. The result is a unified calling environment where end users experience no disruption while the backend architecture evolves.
Use Cases for PRI-Based Connectivity
There are several scenarios in which the Webex to CUBE to PRI model is the preferred choice for organizations adopting cloud communications.
In regions where PRI services are more affordable or where SIP trunks are not widely available or fully regulated, maintaining a PRI-based connection can be more practical. For businesses that require strict call quality assurance or face compliance requirements that mandate circuit-switched telephony, PRI offers a dependable solution.
Organizations in highly regulated industries such as healthcare, finance, or government often have voice recording, logging, or emergency call handling systems that depend on PRI lines. Migrating entirely to SIP without considering these dependencies can pose a risk to compliance or continuity. The Webex to CUBE to PRI model addresses this concern by keeping existing PRI services operational while still enabling access to cloud collaboration.
This model is also ideal for businesses with long-term PRI contracts. Rather than abandoning these agreements and incurring penalties or costs for new IP services, companies can retain their PRI usage while modernizing their collaboration experience in the cloud.
Another scenario involves businesses with centralized call routing. For organizations with multiple branches or satellite offices, calls can be routed centrally to a data center where the PRI circuits are located. Cloud-registered endpoints across all locations can connect through the Webex SIP trunk to a centralized CUBE, which in turn routes calls out through the PRI. This allows for centralized control and monitoring of all PSTN traffic, while still enabling cloud-based collaboration.
Benefits of the Webex to CUBE to PRI Model
One of the key benefits of using PRI for PSTN connectivity is its predictability and stability. PRI circuits are dedicated connections, and call quality is generally consistent due to the nature of circuit-switched technology. For businesses that rely heavily on voice for customer interaction, contact centers, or operational command centers, having a stable and high-quality voice path is critical.
Another advantage is the ability to maintain continuity in operational processes. Because the organization continues to use the same PSTN infrastructure, there is no need to retrain staff, modify upstream carrier agreements, or reconfigure regulatory compliance systems. The transition to the cloud occurs behind the scenes, with the end user still dialing and receiving calls as they always have.
This model also enables simplified failover and survivability options. In cases where cloud access is interrupted or degraded, calls can still be placed and received through the PRI line by local routing mechanisms. The presence of CUBE in the path allows for intelligent call redirection, ensuring minimal service disruption.
Furthermore, Webex to CUBE to PRI provides a strategic pathway for organizations planning a phased migration. It allows IT departments to adopt cloud features incrementally, pilot new workflows, and validate performance before extending the migration across the entire organization.
Limitations and Considerations
While the Webex to CUBE to PRI model offers many advantages, it is not without its limitations. As a legacy technology, PRI circuits are less flexible and scalable compared to SIP trunks. Expanding capacity typically requires new physical connections, and this can be a constraint for growing businesses or those with high call volumes.
PRI services are also increasingly being phased out by telecom providers in favor of all-IP networks. As a result, ongoing support and maintenance for PRI may become more expensive or limited over time. For long-term planning, organizations should assess the lifecycle of their PRI circuits and consider eventual migration to SIP or other IP-based solutions.
The reliance on physical infrastructure can also pose challenges in terms of redundancy and geographic flexibility. If a site with a PRI connection experiences an outage or becomes inaccessible, there may be fewer options for rerouting calls compared to an IP-based solution, which can dynamically redirect traffic across regions or data centers.
There is also a technical overhead associated with maintaining dial plans and routing logic across both cloud and on-prem components. Proper configuration of the SIP trunk, dial-peers, and translation patterns is essential to ensuring calls are routed accurately and efficiently. This may require specialized expertise within the IT or telecom team.
Lastly, some advanced cloud-native calling features may be limited when calls traverse a PRI connection. Features like caller ID manipulation, call analytics, or media optimization may not be available in the same way they are with SIP-based paths. It is important to understand how feature availability is affected by the underlying PSTN connection type.
The Webex to CUBE to PRI connectivity model represents a practical and reliable solution for organizations seeking to transition to cloud collaboration without abandoning their existing telephony infrastructure. It enables businesses to retain their investment in PRI circuits, maintain regulatory compliance, and deliver a consistent calling experience to users, all while embracing the benefits of a cloud-based platform.
This model provides a solid foundation for hybrid deployments, supporting incremental migration and allowing for gradual adoption of new collaboration features. For organizations that prioritize reliability, compatibility, and cost control, the PRI-based approach offers a dependable bridge between legacy systems and the modern Webex Teams environment.
In the series, we will explore the second PSTN connectivity model: Webex to CUBE to SIP. This approach leverages IP-based connections to provide a more scalable and flexible path to cloud-first communications, offering additional advantages for organizations ready to move beyond legacy telephony.
PSTN Connectivity Option 2: Webex to CUBE to SIP
As more organizations shift toward digital transformation and cloud-native technologies, the need for flexible, scalable, and efficient PSTN connectivity becomes increasingly important. While legacy PRI circuits still play a role in certain environments, many enterprises are now adopting SIP-based telephony as their preferred method for connecting to the Public Switched Telephone Network (PSTN). The Webex to CUBE to SIP model is Cisco’s response to this modern requirement, offering a highly adaptable and future-ready architecture for cloud-connected voice services.
This option leverages the same foundational approach as other Webex Teams Calling deployments, where a call originating from a user on a cloud-registered device is evaluated by the Webex Collaboration Cloud’s dial plan. If the dialed number is not recognized as an internal number, the call is routed via SIP trunk to an on-premises Cisco Unified Border Element (CUBE). Unlike the PRI-based model, the CUBE then forwards the call over a SIP trunk to an external IP-based PSTN service provider.
SIP trunks provide an IP-native path for voice traffic and offer many benefits over traditional circuit-switched systems. These include greater flexibility in capacity management, more efficient use of bandwidth, reduced hardware requirements, and better integration with advanced cloud features. With SIP, voice becomes just another data application on the network, making it easier to monitor, secure, and scale.
The Webex to CUBE to SIP model is particularly advantageous for organizations that have already moved away from legacy PRI circuits or are planning to do so shortly. It supports a full range of enterprise calling needs while also enabling businesses to benefit from the agility and cost savings of modern telephony.
Architecture and Call Flow
The call flow in a Webex to CUBE to SIP deployment starts when a user on a Webex Teams-registered device places a call. The Webex cloud checks the internal dial plan to determine if the number matches a known directory number or extension. If no match is found, indicating that the call is destined for the PSTN, it is routed down a SIP trunk to the organization’s CUBE.
CUBE performs the essential role of call translation, security enforcement, and routing control. It examines the dialed number against its configured dial-peers and determines the correct outbound path. In this case, the matching dial-peer forwards the call to an external SIP trunk connected to the organization’s chosen IP PSTN carrier.
Because both the Webex cloud and the PSTN provider use SIP signaling, this model allows for seamless end-to-end call setup without protocol conversion. This reduces latency, improves call setup times, and allows for a more efficient use of network resources.
SIP-based connectivity also enables more intelligent and dynamic call routing. Organizations can use centralized SIP trunking across multiple locations or configure local trunks per site to ensure regional breakout. CUBE supports various advanced features such as load balancing, failover, and media path optimization, allowing businesses to build highly resilient and performant voice networks.
Advantages of SIP Trunk-Based Connectivity
There are several reasons why organizations choose to adopt SIP-based PSTN connectivity in their Webex Teams Calling deployment.
One of the primary benefits is scalability. Unlike PRI circuits, which are limited by a fixed number of channels, SIP trunks can scale up or down based on actual call volume and business needs. This elasticity reduces over-provisioning and makes it easier to accommodate fluctuations in traffic, such as during seasonal demand or unexpected events.
SIP trunks also offer significant cost savings. Most service providers offer more competitive pricing for SIP-based services compared to traditional telephony. Additionally, SIP trunks eliminate the need for separate voice circuits, allowing voice and data to share the same physical infrastructure. This convergence leads to lower operational expenses and simplified network management.
Another major advantage is geographic flexibility. SIP trunks can be provisioned in multiple regions and managed centrally, enabling businesses to deploy consistent calling services across all locations without the need for local PSTN contracts. This global reach is especially valuable for multinational organizations that require consistent communication standards and policies.
SIP connectivity also unlocks access to advanced features that may not be supported over PRI. These include high-definition voice, video calling, advanced call analytics, flexible caller ID manipulation, and support for APIs and integrations. By leveraging SIP, organizations can offer richer and more versatile communication experiences to their users.
Disaster recovery and high availability are also more straightforward with SIP. Redundant trunks can be configured, and CUBE can automatically reroute calls in the event of failure. SIP’s flexibility in routing and its compatibility with modern session border controllers and firewalls make it easier to design resilient architectures that align with business continuity goals.
Considerations for SIP Trunk Deployment
While SIP trunking offers many benefits, it also introduces new technical and operational requirements that organizations must carefully manage. Deploying the Webex to CUBE to SIP model requires a solid understanding of SIP protocols, security best practices, and network design principles.
One of the most critical considerations is ensuring Quality of Service (QoS) across the network. Because voice traffic now shares the same infrastructure as data, there must be sufficient bandwidth, traffic prioritization, and latency management to maintain consistent call quality. IT teams must configure appropriate QoS policies across routers, switches, and firewalls to prevent jitter, delay, and packet loss.
Security is another key area of focus. SIP traffic should be encrypted using protocols such as TLS for signaling and SRTP for media. CUBE provides robust security features including encryption, media anchoring, access control, and traffic policing. It is important to work closely with both the internal security team and the SIP trunk provider to ensure end-to-end protection.
Choosing the right SIP trunk provider is also essential. Providers differ in terms of coverage, feature sets, support models, and pricing structures. Organizations should evaluate vendors based on technical compatibility, regulatory compliance, porting capabilities, and support for emergency services and lawful intercept.
Interoperability between the SIP trunk provider and CUBE must be validated. Differences in SIP dialects and headers can cause call setup issues, codec mismatches, or feature limitations. Cisco provides detailed interoperability testing and configuration guides for many leading SIP providers to streamline deployment.
Licensing and endpoint configuration must also be considered. Devices must be licensed appropriately under the Cisco Collaboration Flex Plan, and endpoints must support cloud registration and SIP signaling. Admins must also ensure that dial plans are properly coordinated between the Webex cloud and CUBE to avoid misrouted or failed calls.
Use Cases for Webex to CUBE to SIP
This connectivity model is ideally suited for businesses that are embracing a cloud-first strategy and want to modernize their voice infrastructure. It allows for full utilization of the Webex platform’s cloud calling capabilities while maintaining direct control over PSTN routing and policies.
Technology companies, digital service providers, and enterprises with dynamic and distributed workforces benefit from SIP’s flexibility and global reach. They can provision and manage numbers across multiple countries, integrate calling with customer engagement platforms, and scale rapidly in response to changing demand.
Organizations looking to reduce infrastructure complexity and consolidate services onto a single IP network also benefit from SIP trunking. It eliminates the need for separate voice and data networks and simplifies ongoing maintenance and monitoring.
This model also supports innovation. Because SIP is standards-based and widely adopted, it opens the door to integrations with third-party platforms, custom development, and advanced analytics. Businesses can build solutions that are deeply integrated into their workflows, from CRM integration to contact center optimization.
Finally, SIP trunking supports sustainability and modernization initiatives. Moving away from legacy TDM circuits reduces hardware dependencies, enables more efficient power consumption, and aligns with green IT strategies.
The Webex to CUBE to SIP model represents the next stage in enterprise telephony evolution. It enables organizations to transition fully or partially to cloud-based collaboration while leveraging the flexibility, efficiency, and power of SIP-based PSTN connectivity.
This model is well-suited to modern businesses that prioritize scalability, agility, and cost efficiency. It allows IT teams to build future-ready communication architectures that support global operations, remote work, and advanced collaboration scenarios.
By combining the Webex Collaboration Cloud with Cisco’s CUBE and a trusted SIP provider, organizations gain the tools and infrastructure needed to deliver high-quality, cloud-enabled calling without compromising on control, performance, or user experience.
In the series, we will explore the third PSTN connectivity option: Webex to CUBE to PBX to PSTN. This model is particularly relevant for businesses that still rely on traditional PBX systems and want to integrate cloud calling without disrupting their existing call flows and configurations.
PSTN Connectivity Option 3: Webex to CUBE to PBX to PSTN
In environments where legacy telephony infrastructure is still deeply embedded in business operations, transitioning to cloud calling platforms must be approached with precision and flexibility. Many organizations are not yet ready to retire their on-premises PBX systems due to custom integrations, specialized dial plans, regulatory obligations, or ongoing business processes that rely on existing hardware and workflows. The Webex to CUBE to PBX to PSTN model addresses these challenges by enabling enterprises to adopt cloud calling through Webex Teams while still leveraging their existing PBX systems and associated PSTN connections.
This connectivity model provides a hybrid architecture that allows cloud-registered endpoints to interact with on-premise PBXs for call routing. It is particularly useful for businesses that want to migrate a subset of users to Webex Teams while keeping others on the traditional PBX. It also supports use cases where specialized devices or operational systems are still dependent on the PBX for internal routing or outbound calling.
Architecture and Call Flow
The call flow in this model follows a structured path from cloud to PSTN via multiple on-prem components. A user initiates a call using a device registered to the Webex Collaboration Cloud. The dialed number is evaluated against the Webex internal dial plan. If there is no match for the number within the Webex cloud, the call is sent over the SIP trunk to the on-premises Cisco Unified Border Element (CUBE).
Once the call reaches the CUBE, it is evaluated based on configured dial-peers. Instead of being routed directly to a PRI circuit or SIP trunk, the call is forwarded to the organization’s on-premise PBX. The PBX then processes the call according to its dial plan. If the number corresponds to a user or device registered to the PBX, the call is routed internally. If the number is external, the PBX sends the call to the PSTN using its existing PRI or SIP connection.
This layered call processing approach ensures that the PBX retains control over outbound and internal call routing, while the Webex Teams user still benefits from cloud-based calling features and collaboration tools. It allows both systems to coexist and interoperate, creating a seamless user experience despite the complexity of the underlying architecture.
Benefits of PBX Integration
The primary benefit of integrating Webex Teams Calling with an on-premise PBX is the ability to preserve and extend existing telephony investments. Many organizations have spent years building custom PBX environments that include automated attendants, call queues, extension dialing, departmental routing, and integration with other business systems. Rebuilding this logic in a cloud platform may not be practical or desirable in the short term.
By leveraging the existing PBX, businesses can continue to use familiar workflows while gradually introducing Webex Teams to select users or departments. This enables a staged migration that reduces disruption and simplifies change management. As confidence in the cloud platform grows, additional users can be migrated, and PBX functionality can be replicated or replaced as needed.
The model also supports highly customized call flows. For example, a support desk or customer service team may require calls to flow through a specific PBX call center application. Webex Teams users can be configured to pass calls to that PBX infrastructure, preserving existing processes while adopting new collaboration capabilities.
This model also allows for unified dial plans across cloud and on-prem users. With proper configuration, users on both platforms can dial each other using internal extensions, even though they are registered to different systems. CUBE and the PBX work together to ensure that dialed digits are translated and routed accurately.
Another key advantage is compliance. Some industries have stringent regulatory requirements around call handling, recording, or data retention that are already embedded in PBX systems. This model allows businesses to continue using those compliant systems while beginning their cloud migration journey.
Deployment Scenarios
The Webex to CUBE to PBX to PSTN model is suitable for a variety of enterprise use cases.
Organizations with mission-critical PBX-dependent operations, such as healthcare providers with nurse call systems, universities with building intercoms, or manufacturing facilities with paging systems, often find it difficult to move entirely to the cloud. This model allows such organizations to modernize communication for office-based staff through Webex Teams, while keeping operational staff on traditional systems.
Enterprises with a global footprint may also choose to retain PBX systems in some regions due to local compliance, support agreements, or hardware dependencies. Webex Teams can be deployed centrally or in select regions, while the PBX continues to serve other areas. This allows for a flexible, regionally tailored migration approach.
The model is also valuable during transition periods such as mergers, acquisitions, or office consolidations. It enables multiple systems to function in parallel while a long-term integration plan is developed. Employees on newly acquired systems can be onboarded to Webex Teams, while legacy PBXs are phased out at a controlled pace.
Companies running hybrid office models or supporting a mix of remote and on-site users may also benefit. Remote users can be connected to the Webex cloud, while on-site users continue to utilize local PBXs. This preserves connectivity across both environments without the need for an immediate system overhaul.
Technical Considerations
Implementing the Webex to CUBE to PBX to PSTN model requires careful planning and coordination across multiple technology layers.
Dial plan consistency is one of the most critical factors. The dial plans used in Webex, CUBE, and the PBX must be aligned to ensure accurate call routing and user reachability. Mismatched digit lengths, prefixing, or translation rules can cause failed or misrouted calls. Dial plan coordination is especially important in environments where both cloud and on-prem users need to call each other using internal extensions.
Call routing logic within CUBE must be configured to recognize which calls should be forwarded to the PBX versus those that should go directly to the PSTN. Dial-peer selection, digit stripping or appending, and codec configuration all play a role in achieving the correct call flow. CUBE’s routing engine must be aware of the PBX’s numbering scheme and capable of handling the associated call paths.
Security is another consideration. The SIP trunks between Webex, CUBE, and the PBX must be protected using encryption and access control policies. CUBE provides mechanisms to secure signaling and media, inspect SIP headers, and enforce session policies. Organizations must also ensure that their PBX systems are updated and configured to prevent toll fraud, spoofing, or denial-of-service attacks.
Interoperability between CUBE and the PBX must also be tested. Differences in SIP implementations can lead to issues with call setup, codec negotiation, or supplementary services like call hold, transfer, or conferencing. Ensuring both systems can communicate effectively is critical to providing a reliable user experience.
Resource planning is also important. Media processing, transcoding, and signaling handling can place demands on CUBE, particularly in high-volume environments. Capacity planning should include headroom for peak traffic and redundancy in case of hardware or network failure.
Challenges and Limitations
While the Webex to CUBE to PBX to PSTN model offers great flexibility, it also introduces certain challenges.
Managing and maintaining multiple systems increases complexity. The coexistence of Webex, CUBE, and a legacy PBX requires expertise across multiple platforms. Troubleshooting call issues can involve coordination between different teams and systems, leading to increased time to resolution.
Another challenge is the potential for feature disparity between cloud and PBX users. While Webex Teams offers modern collaboration features like persistent messaging, video meetings, and AI-driven call insights, PBX users may remain limited to basic telephony. This can create uneven user experiences and may require training or process adjustments to align workflows.
Some advanced Webex features may not function fully when calls are routed through a PBX. Call analytics, quality monitoring, or cloud-based call control features may have limited visibility into PBX-handled calls. Organizations must assess which features are critical to their business and whether they are supported in hybrid call paths.
There is also the risk of delaying full cloud migration by maintaining PBX dependencies. While integration provides a bridge, over-reliance on the PBX can slow down innovation, increase support costs, and lock the organization into legacy systems longer than necessary. Strategic planning should include milestones for gradually decommissioning the PBX over time.
Final Thoughts
The Webex to CUBE to PBX to PSTN model offers a powerful and flexible option for enterprises that require hybrid telephony solutions. It enables businesses to adopt Webex Teams Calling while continuing to leverage their existing PBX systems for call processing and PSTN connectivity.
This model is especially useful for complex, regulated, or specialized environments where a complete migration to the cloud is not feasible in the short term. It supports phased transitions, minimizes disruption, and preserves critical voice services while enabling access to modern collaboration tools.
By carefully planning dial plan alignment, routing logic, security, and interoperability, organizations can create a cohesive hybrid architecture that supports current needs and positions them for future migration. As the business evolves and dependencies on legacy systems decrease, the integration can be simplified, leading to a more fully cloud-based collaboration environment.
With all three PSTN connectivity models—PRI, SIP, and PBX integration—Cisco’s Webex Teams Calling offers unmatched flexibility for enterprises navigating the transition to cloud communications.