Hi Ajinkya…,
May be there are lot things involved in this problem …
1.MSC-CUG Capability
2.ISUP-InterOperatability
3.Analyze what kind of Switches supports CUG Calls in other network.
4.At the exchnge where the calls are failing trace the outgoing calls for that CUG Member and same with the exchnges where it succeeds.
5.Check the incoming message from another cellular network.
Rgds