Core Issue
In a multi-clustered environment with multiple subscribers in a cluster, where the Intercluster Trunk (ICT) is set up between the publishers of both clusters, some of the calls between the clusters fail.
For example, assume there are two sites and clusters in an enterprise (Houston and Mexico). Mexico has one publisher and one subscriber, whereas Houston has only one publisher.
Mexico's publisher and Houston's publisher are connected with an ICT. If a device registered to Mexico's publisher calls any device in Houston and if a device registered to Houston's publisher calls any device in Mexico, the call goes through. However, when any device registered to Mexico's subscriber makes a call to Houston and when any device registered to Houston's publisher makes a call to Mexico, the calls fail.
Resolution
To resolve this problem, set up one more ICT between Mexico's subscriber and Houston's publisher.
In an enterprise where there are multiple subscribers within a cluster, this solution does not scale well because you end up configuring too many ICTs. In such cases, configure a pseudo-gatekeeper, as described in Setting up Inter-cluster Trunks with Three or More Cisco CallManagers.
For more information, refer to Calls to IP phones across an Inter-Cluster Trunk fail with a reorder tone or busy tone
