When making inter-cluster calls, if the called number is not present in the first route group or first device in the route group, then the call fails with unassigned/undefined number. This happens even though the number is configured in the other Cisco CallManager cluster which can be reached from the next route group.
This issue occurs because the Stop Routing on Unallocated Number Flag service parameter is set to True by default in Cisco CallManager 3.3 and later. This service parameter is set to False in Cisco CallManager versions earlier than 3.3. When calls do not route out of the additional gateway, even though the route group is configured correctly, the likely cause is that the Stop Routing on Unallocated Number Flag service parameter is set to True.
The Stop Routing on Unallocated Number Flag service parameter is only valid for inter-cluster trunk calls. When this flag is set to True and a call is routed to a remote Cisco CallManager cluster through a route list and is released by a remote Cisco CallManager because of the un-allocated number, then the local Cisco CallManager stops routing the call to the next device of the route list. When the flag is set to False, the local Cisco CallManager routes the call to the next device. An un-allocated number is a dialed directory number that does not exist in the Cisco CallManager cluster.
Set the Cisco CallManager Stop Routing on Unallocated Number Flag service parameter to False to resolve this issue. Select Service > Service parameters > Select a server > Select Cisco CallManager as the service > Cluster wide Parameters (Route Plan) to set this parameter. This makes the route group behave as expected. Make sure that all of the Cisco CallManager servers in the cluster have the same value.
Note: If a User Busy is reported by the gateway and it is unable to route calls, set Stop Routing on User Busy Flag to False in order to solve the issue.