Take a packet capture of a failed call. (If you use the MBG, it's easy through the MBG to kick off the packet capture).
Then open the capture in Wireshark, click "Telephony", then "VOIP Call" then select the call and hit "Flow Sequence". This should help you see which part of the SIP negotiation leads to the error.
The kinds of things you can look for are,
CODEC=G711, ptimer=20, t38=off