3.9 Implementing PSTN Backup for On-Net Intersite Calls

This topic describes how to use the PSTN as a backup for on-net intersite calls.

If the IP WAN (ICT) fails, calls are rerouted over the PSTN:

  • 8 (site code).XXXX route pattern per site
  • Route pattern points to route lists (first option: ICT; second option: local route group)

The figure shows a multisite deployment with two sites. Each site has its own Cisco Unified Communications Manager cluster. Intersite calls should use the ICT over the IP WAN. However, what if the IP WAN is down? Because both sites have access to the PSTN, the PSTN should be used as a backup for intersite calls.

To ensure that phones at different sites always use their local gateway for PSTN backup, you can configure a route list that includes the ICT as the first option and the local route group as the second option. With this approach, there is no need to have multiple, site-specific route lists with a different, site-specific route group as the second entry. When the primary and the backup path require the called numbers to be formatted differently, such as in the example, where you use internal site-code dialing over the ICT and PSTN numbering for the PSTN backup path, the called number has to be modified, depending on the chosen path.

Digit Manipulation Requirements for PSTN Backup of On-Net Intersite Calls

PSTN backup for on-net calls can be easily provided by route lists and route groups that give priority to the ICT instead of the PSTN gateway.

When you use PSTN backup for on-net calls, you must address internal versus external dialing. While on-net calls usually use site codes and directory numbers, calls that are sent through the PSTN have to use PSTN format. Digit manipulation requirements vary, depending on the path that is taken for the call:

  • Digit manipulation requirements when you use the ICT (first choice in route list and route group):
    1. At the calling site: The access and site codes are added to the calling number.
    2. At the receiving site: The access and site codes are removed from the called number.
  • Digit manipulation requirements when you use the PSTN (secondary choice in route list and route group):
    1. At the calling site: The internal called number, which comprises an access code, site code, and directory number, is transformed to the PSTN number of the called phone. The calling number is transformed to the PSTN number of the calling phone.
      Note

      If DID is not supported, the PSTN number of the site, rather than the PSTN number of the IP phone, is used in called number and calling number.

      When you require a different digit manipulation configuration, depending on the selected path, you can configure the digit manipulation settings either at a path-specific route group or by using global transformations at the egress device.

    2. At the receiving site: The PSTN calling number is recognized as a PSTN number of an on-net connected site and transformed to the internal number: access and site codes, followed by the directory number of the calling phone (if DID is used at the calling site) or of the attendant of the calling site (if DID is not used at the calling site). The called number is transformed to an internal directory number and routed to the IP phone (if DID is used at the receiving site) or to an attendant (if DID is not used at the receiving site).