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= 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
destination-pattern 60.
session protocol sipv2
session target ipv4:
session transport tcp
dtmf-relay rtp-nte
audio incoming level-adjustment -5
audio outgoing level-adjustment -15
codec g711ulaw
no vad

Other things to check would be codec changes or TCP/UDP changes as either would cause transcoding.

Update:  I ran into a similar error again.  This time deleting the dial-peer and re-adding it, along with restarting the SIP services, fixed the problem.