Where to start:
SBC tracing (wireshark or built in tools)
Session Manager: traceSM
PBX - I'm assuming Avaya CM, so "list tra tac XXX"
Is this internal calls getting dropped, or just external? If it's external, that simplifies things. Just start from the inside, troubleshoot CM <->SM then SM<->SBC and see who drops the call or audio/RTP and when.