For starters pots are ok to 9600 baud. anything after that is a freebee. The pri's are 64k channels that have to be broken down for voice. Some time back there was a post on channels and why modems and faxes dont work well on them. I dont know were it is now though. Either way it's mute.

The problem is that the fax is not negotiating at the speed it thinks it is were as the other end just keeps resending thinking its negotiating at a constant speed. Its a break down in two places. first the system then the co were the pri is converted twice.