We changed a customer from PRI to SIP trunk and after the change, the Exchange UM stopped working for calls coming in from the outside. We found the following error in the logs:
%VOICE_IEC-3-GW: CCAPI: Internal Error (Software Error): IEC=188.8.131.52.13.112 on callID 78
Some posts mentioned upgrading the firmware (which we did with no effect). The dial-peer pointed to Exchange had some volume adjustments on it. Once we removed the adjustments, the error went away and calls went through.
dial-peer voice 100 voip
description Dial-peer for Exchange VM
session protocol sipv2
session target ipv4:10.5.99.11:5065
session transport tcp
audio incoming level-adjustment -5
audio outgoing level-adjustment -15
Other things to check would be codec changes or TCP/UDP changes as either would cause transcoding.