I would suggest looking at the Media (SDP) negotiation...
Without knowing the details, the symptom makes me instantly think that the Media is not being negotiated correctly, therefore the disconnect occurs.
The call is being established, so the SIP signaling seems to be fine. But when you answer the call on Communicator, the Media needs to be correctly negotiated for that call to remain active and pass audio end-to-end.
An ethernet sniff, would show you this quite quickly (and it would show you who is initiating the disconnect)
Good Luck
MF