A Cisco Unified SRST gateway does not need to advertise any internal directory numbers because the SRST site is reachable only via the PSTN. It is the responsibility of Cisco Unified Communications Manager to know how to route calls to cluster-internal directory numbers when they are not reachable over the IP WAN.
However, the Cisco Unified SRST gateway needs a local dial plan that allows end users to place calls to other sites by dialing the internal directory number of the other site. Cisco Unified SRST then must transform the internally used directory numbers to the corresponding PSTN numbers so that the call can be rerouted over the PSTN. You do not have to configure this local dial plan manually when CCD is used. Instead, Cisco Unified SRST can subscribe to SAF and hence learn all internally known DN ranges and the corresponding ToDID rules. Cisco Unified SRST learns these routes while there is no network problem. The learned patterns are not used then because the Cisco Unified SRST gateway does not route any calls. Cisco Unified Communications Manager is in control of all IP phones and performs call-routing services.
Once IP connectivity is broken, IP phones fall back to the Cisco Unified SRST gateway, and once registered, the Cisco Unified SRST gateway must route calls. Because the gateway has learned all available internally used directory numbers with the corresponding ToDID rules, it can now route to the respective PSTN number any calls that are based on the dialed internal directory number.
In the example, the Cisco Unified SRST gateway learned three patterns while IP connectivity was working: 8408XXXX with a ToDID rule of 4:+1408555, 8415XXXX with a ToDID rule of 4:+1415777, and 8949XXXX with a ToDID rule of 4:+1949222.
When a user dials 89491000 while the gateway is in SRST mode, the IP path is marked as unreachable due to the loss of IP connectivity. Therefore, the ToDID rule is applied. It strips the first four digits and then adds the prefix +1949222 so that the number that is used for the PSTN backup call is +19492221000.
The only static configuration that is required at the Cisco Unified SRST gateway is an outbound dial peer that routes calls starting with a plus sign (+) toward the PSTN.