It seems like Brother implemented the fax modem differently for each printer. On the MFC 4600 the fax modem was a fax modem. The MFC 6800 modem uses the interface to setup the machine. What I mean is that Brother is using a vender specific AT command set to get information from the brother MFC. The user can basically get the current settings and send new settings using the AT command set.
On the 6800, the fax modem does not work at all. There might be a init string that can be sent to the modem to get it to work but I don't know what it is.