In a situation where the main site uses Cisco CallManager 4.x and with a remote site uses Cisco CallManager Express, the gatekeepr trunk is the only tie between the two systems. It is possible that there is a requirement to use Automated alternate routing (AAR) to reroute calls that go through a gatekeeper WAN trunk if the gatekeeper is not able to process the call.
The AAR provides a mechanism to reroute calls through the PSTN or other network by using an alternate number. As a subset of the AAR feature, Cisco CallManager automatically reroutes calls through the PSTN or other networks when Cisco CallManager blocks a call due to insufficient location bandwidth. With Automated Alternate Routing, the caller does not need to hang up and redial the called party. So, basically, if you have a bandwidth limitation between sites, you can utilize this feature.
The AAR is tied in with the Cisco CallManager Call Admission Control and is used for devices that are registered to the Cisco CallManager such as phones. If routing a call through the public switched telephone network (PSTN) is required as a secondary option, and if the gatekeeper rejects the call, then the PSTN gateway can possibly be specified as a second route group, with its own Called and Calling number modifications within a Route List.
The modifications made to the Called and Calling Numbers under a Route List override any modifications made at the Route Pattern