That sounds like an odd issue... What phone system, can we assume this is a 3300 (MiVoice Business or MCD)? Is this a new issue where calls were working, or this an issue with a new trunking service?
Personally, I would look at the traces yourself... In the MBG you can do a packet capture, start the capture and place a test call, then download the packet capture file and open it in Wireshark and go to Telephony - VoIP calls and look at the FROM and TO fields, or filter based on that call and look at the INVITE packets in the SIP section to verify what you are sending to the ISP.