I have tested IP Communicator versions 2.1(1), 2.1(4) and 7.0(1). CUCMBE version is 126.96.36.1990-13.
First call is placed OK. After closing the call, if I place a second call inmediately no audio in both direction results. If I made it successively I get same behavior. But if I wait 2 to 3 minutes after closing a call, next call works fine again.
Problem happens when calling to another IP Communicator or physical phone on the LAN. When calling to PSTN through a 3845 with E1 interface all calls work fine.