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.
To resolve this problem, set up one more ICT between Mexico's subscriber and Houston's publisher.