PSTN trunking enables users of organizations migrating to Webex Calling to communicate externally without the need for multiple PSTN contracts or complex routing setup.

Benefits of PSTN Trunking

PSTN trunking offers the following benefits to your organization:

  • Cloud PSTN migration before user migration—Organizations can transition their PSTN connectivity to the cloud before moving users to Webex Calling. This phased approach results in simplifying network architecture and lowering the cost.

  • Reduced infrastructure costs—Eliminates the need for multiple on-premises gateways and PSTN circuits, simplifying network architecture that lowers the cost.

  • Simplified integration with vertical applications—Makes it easier to connect specialized applications, such as financial turrets, nurse call system, and others, directly to the cloud PSTN without complex configurations.

  • Unified dial plan—Consistent extension and dial plan management across all sites and applications.

Key features of PSTN trunking

  • Supported PSTN types are:

    • Cloud Connected Partner PSTN (CCPP)

    • On-premises PSTN with Local Gateway

  • Direct PSTN access—On-premises PBX users can place and receive PSTN calls through the cloud PSTN via Webex Calling.

  • Caller ID features are:

    • Caller ID delivery and blocking

    • Calling number and name delivery

  • Emergency calling—Emergency calling from on-premises users using cloud PSTN.

  • Redundancy and Failover support—Route groups combine multiple trunks to allow for multiple routes between on-premises and cloud PSTN. Calls are automatically rerouted if one trunk connection fails.

Webex Calling Route List Calls License

You need an add-on Webex Calling Route List Calls license to enable the PSTN trunking feature. The number of licenses you buy sets the limit for concurrent off-net (PSTN) Route List calls allowed between your on-premises system and either cloud or on-premises PSTN.

The add-on license automatically includes 20% bursting beyond your licensed call volume, which allows your organization to handle occasional increases in call traffic without service interruption.

Table 1. Route List Calls license details:

Customer type

Included concurrent Webex Calling Route List Calls licenses

How to add more capacity

Bursting policy

Notifications and monitoring

Enterprise Agreement

25% of licensed users

Using change order, you can purchase Add Route List SKU for an existing subscription.

20% bursting, then non-emergency calls are blocked.

Alert on Control Hub when call is blocked beyond bursting capacity. CDR includes fields to show bursting and blocked calls.

Named User

Not included.

Using change order, you can purchase Add Route List SKU for an existing subscription.

20% bursting, then non-emergency calls are blocked.

Alert on Control Hub when call is blocked beyond bursting capacity. CDR includes fields to show bursting and blocked calls.

Trial

10 concurrent calls

Not Applicable

20% bursting, then non-emergency calls are blocked.

Alert on Control Hub when call is blocked beyond bursting capacity. CDR includes fields to show bursting and blocked calls.

Existing Enterprise Agreement customers need to do a subscription modification to get Route List Calls licenses.

How PSTN trunking works

Architecture overview

PSTN trunking uses Route Lists in Webex Calling to integrate your on-premises PBX system, or any specialized vertical applications, with Cloud Connected Partner PSTN (CCPP) or local gateway based PSTN.

Route Lists in Webex Calling are lists of numbers reachable through a Route Group. Each Route List is exclusively assigned to a location that supports up to 40,000 unassigned numbers from the hosted pool.

The following diagram illustrates the high-level architecture of PSTN trunking. The Enterprise Session Border Controller (SBC) terminates the Route Groups from Webex Calling. You can use any of the SBCs supported by Webex Calling.

A network diagram showing the call flow paths originating from "Webex Calling"

Call flows

  • Inbound Calls—External calls from the PSTN are routed on one of the trunks associated with the Route Group, if the called number matches a number configured in the Route List.

  • Outbound Calls—Outgoing calls from an on-premises user are routed on the PSTN trunk associated with the location, when the calling number matches a number configured in the Route List. Route List can also make internal (in-org) calls.

Emergency Calling

If you enable RedSky for the route list's location, the emergency calls from that route list is routed through RedSky. You must map the route list number in the RedSky portal.

If RedSky is not enabled for the route list location, emergency calls will route through location's PSTN provider. See article Enhanced emergency calling for Webex Calling.

How to enable PSTN trunking

This article shows provision flow for enabling PSTN trunking for on-premises PBX and for integration with vertical application.

Before you begin

  • The calling party number in the SIP INVITE from on-premises system to Webex Calling must match the E164 number configured in the Webex Calling route list for the call to succeed.

  • Review the Audio specifications for Webex Calling documentation for supported/preferred audio codec by Webex Calling. This must be considered for the design of PSTN trunking for hybrid Webex Calling deployments.

1

Sign in to Control Hub.

2

Provision Location(s) in Control Hub.

3

Select Cloud Connected PSTN as the connection type for the location(s) and select the corresponding CCP provider(s).

Webex Control Hub screen for selecting PSTN connection type, with Cloud Connected PSTN selected.
4

Order PSTN numbers from a CCP provider (integrated or non-integrated).

Integrated providers will supply numbers directly to Control Hub, where they will appear automatically. For non-integrated providers, import the PSTN numbers as follows:

  1. In Control Hub and go to Calling > Numbers menu.

  2. Click the Manage drop down menu on the right side of the table and select Add.

    Add phone number in Webex Control Hub Calling section

  3. Select the Location from the drop-down menu on the selection page.

    The PSTN connection associated with the location is listed against the location.

  4. Add the PSTN numbers purchased in the fields on the Select Numbers page.

    You may add up to 1000 numbers and choose to activate them immediately or later. Click Save after all numbers have been added.
    Enter and activate phone numbers in Webex Control Hub, with options to format and save up to 1000 numbers.
    The confirmation page displays the PSTN numbers that have been added to the location.
5

Perform the following steps for each location to create a Route List, assign it to an appropriate Route Group, and select which numbers will be used on-premises.

  1. Go to Calling > Route Lists and choose one from the list to view its properties.

  2. Select a Route Group from the Routing Choice menu.

  3. Click Add Numbers and enter the numbers associated with the Route List.

    Webex Control Hub Route List ‘RL-SJC’ shows 6 phone numbers assigned to the location SJC, with options to add or import numbers

  4. Select the PSTN numbers in the Route List that are designated for use on-premises and click Add.

    Webex window to add numbers to RL-SJC route list; 1 of 2 numbers selected
6

(Optional) If there are on-premises numbers in addition to route list numbers, configure Enterprise Dial Plans in Webex Calling with patterns pointing to on-premises and associate it with a Route Group.

7

Configure on-premises call handling:

  1. Configure the PSTN DIDs and assign them to phones, users, and hunt pilot as required.

  2. Configure dial plan on-premises to route PSTN calls to Webex Calling, using the Route Group, Route List, and SIP trunks configured during on-premises service activation.

8

To enable international calling, select the relevant location in the Control Hub > Calling page.

  1. Navigate to the Advanced > Outgoing and Incoming Permissions > Outgoing Calls > International.

  2. Select Allow from the drop-down menu.

PSTN trunking usage metrics

The existing Route List page in Control Hub is enhanced to give administrators real-time visibility into active off-net calls across all numbers in the Route List, along with insights into peak concurrent call volumes over the past 72 hours. By translating call usage into clear capacity trends, it enables proactive planning to avoid costly over-provisioning. The Route List page call usage metrics ensures that every call gets through that enhances user experience and ensures that the organization can scale seamlessly to meet business growth. You can also view call usage metrics of all the trunks in Trunk page in Control Hub.

What happens when call volume exceeds Route List Calls license limit?

When the number of concurrent calls exceeds the Route List Calls license limit, up to 20% additional capacity (bursting) is allowed. If off-net calls occur during this bursting period, the originating on‑premises user hears an announcement informing them that the call is in bursting mode and advising them to contact their administrator to help prevent future off‑net calls from being blocked. The announcement is only played for outbound off-net calls (excluding emergency calls), not for inbound calls.

When the off-net call volume exceeds 20% bursting limits, they are blocked. In case of outbound off-net calls (excluding emergency calls), the caller hears an announcement that the call is blocked since it exceeds the Route List Calls license limit. Incoming calls are rejected but caller does not hear this announcement.

The Blocked call system alert is added to "Alert center" in Control Hub, it appears as:

  • Service—Calling

  • Type—Route List Calls Blocked

  • Severity—High

  • Description—A PSTN call was blocked because the number of simultaneous calls exceeded the license limit for all Route List numbers. Review your organization's Route List Calls licenses. Consider increasing your Route List Calls license quantity to prevent future disruptions.

You can enable alerts for delivery in Control Hub and also select the delivery channels such as Email, Webex space, Webhook, and PagerDuty.

CDR changes

These are the changes to the Webex Calling CDR introduced by PSTN trunking:

  • Route List Calls Overage—This new field is added to CDR. This is triggered when an off‑net Route List call exceeds the organization’s Route List Calls license limit. It shows the total number of calls, including this one, that were in bursting at the time the call was made or received.

  • Call Outcome Reason—The Call Outcome Reason field has a new RouteListCalls value added. It’s set whenever an off-net route list call is blocked because the organization has exceeded its Route List Calls license limit.

PSTN on-premises migration scenario

As a part of their migration from Cisco Unified Communications Manager (CUCM) to Webex Calling, customer wants to migrate PSTN to Cloud Connected Partner PSTN (CCPP) to reduce infrastructure cost by eliminating the need for multiple on-premises gateways to maintain PSTN connection. By using PSTN trunking, their on‑premises users can use CCPP PSTN without having to migrate to Webex Calling. This phased approach allowed the organization to transition users in stages while minimizing the risk.

To use PSTN trunking for the migration, the customer migrates their existing PSTN service from on-premises to cloud PSTN, adds E.164 numbers in Control Hub. The customer administrator follows the instructions in Provisioning Flow to enable PSTN Trunking for On-Premises PBX and for Integration with vertical applications to configure Route List on Webex Calling.

Configuration on Local Gateway/SBC:

Follow instructions in this article, Get started with Local Gateway to configure local gateway.

Configuration on Cisco Unified Communications Manager (CUCM):

The following are the configuration steps on CUCM:

  1. Go to Call Routing > Directory Number, to configure the E.164 numbers that you ordered from PSTN in CUCM.

  2. Assign these numbers to devices of the migrated users.

  3. Create a SIP trunk on CUCM that connects to the Local Gateway.

  4. Configure a Route Group. Add the SIP trunk created in the previous step to this Route Group.

  5. Configure a Route List and add the Route Group to this list.

  6. Configure the Route Pattern for the PSTN calls. Assign to the Partition accessible only by users allowed to use Cloud PSTN.

  7. Set Up Calling Search Space (CSS) for migration.

    1. Create a CSS that includes the partition containing PSTN Route Pattern.

    2. Apply this CSS to phones or device profiles of migrated CUCM users, so their PSTN calls routes to Webex through the Local Gateway.