The one that always gets me at least 2-3 times a year... "Dial In Trunks Incoming Digit Modification - Absorb" in Trunk Attributes must NOT be blank.
Make sure you have your DID's in the SIP Peer Profile Assignment by Incoming DID form.
Otherwise, as NEPhoneGuy stated... run a packet capture from the MGB and make a call that fails, then go back and look at the traces in Wireshark.