This all depends on the Fax interface. T1, digital or analog. You can set up an ISDN custom protocol Tie trunk using network or pbx switch types as long as the fax is the oppisite (host-peer). This would be rare in fax servers. Probably you would use a E&M Tie trunk that will re-DTMF the digits accross the T-1 using the UDP routing tables. Think of the Fax server as a networked PBX. Most fax servers work this way or just use analog stations in a non-integrated VM hunt-group. You renumber adjunts to = your fax DIDs and cover them to the huntgroup The fax software will need to learn or know the Legend mode-codes to route the calls.