I don't know about incoming faxes, but for outgoing, I found a solution...
It appears that there is not enough time when the two fax machines "handshake" at the beginning of the call. When I fax out, enter the phone number and place 5 or 7 "," at the end of the phone number. The comma's act as a pause. This gives the two machines enough time to "handshake" and establish a good connection before transmitting data.
So if you have a phone number of 123-456-7890 make it 123-456-7890,,,,,,,
Try it again and see if that helps...