Each UCM will need a set of inbound and outbound rules in order to allow calls between site extension.
If UCM A is using extensions in the 4XX range and UCM is using extensions in the 1XX and 2XX range, then
UCM A will need an outbound rule to allow for dialing 1XX and 2XX using the peer trunk to UCM B
UCM A will need an inbound rule to allow for 4XX from UCM B (peer trunk) using “by DID”
UCM B will need an outbound rule to allow for dialing 4XX using the peer trunk to UCM A
UCM B will need an inbound rule to allow for 1XX and 2XX from UCM A (peer trunk) using “by DID”
The extensions at both UCMs should not conflict with any other extension in the system. In other words, if you are trying to dial user extension 4XX from UCM B, there should not be any other extension of any type using the same 4XX extension. For example, If there is an IVR with extension 4XX in UCM B, the call will always go to the IVR rather than to the remote extension. The UCM will always try and match internal extensions before checking the outbound rules.