Core Issue
After a Media Gateway Control Protocol (MGCP) channel-associated signaling (CAS) call is established, there may not be voice-path continuity; the call signaling is properly terminated, but there is only one-way voice traffic.
Resolution
To work around this issue reconfigure the gateway to make use of H.323 instead of MGCP.
To resolve this issue, upgrade to one of the following versions 12.3(1), 12.2(15)T02, 12.3(1.3), 12.3(1.3)T.
Bug ID# CSCin40652 documents this issue.