Does it sound as if the faxes are timing out, or are they negotiating a connection and then disconnecting?
Many faxes have proprietary transmission modes that allow them to transmit faster to the same make/model. It could be that the type of fax that you have, and the Brother All-in-one faxes have similar proprietary modes, so each attempts to negotiate that mode, then fails when they aren't exactly the same.
Is your fax a Brother as well?
If you can, attempt to configure your machine to connect as either a Group II or at 9600 bps or slower (7200 or 4800). The faxes could be having issues attempting to connect at the Group III mode or 14.4k rate.
pansophic