You could try this. Get two telephone sockets and a 12V regulated
power
supply. Make sure the power supply is *not* earthed. Wire the
positive
terminal of the power supply to one of the active terminals in the
first
socket*, the other active terminal to the second socket, and the other
active
terminal of the second socket to the negative terminal of the power
supply.
Plug your FAX and modem into the two sockets, so they are in series
with one
another and the power supply.
This will allow the modem to 'hear' the tones created by the FAX
machine, but
it is as though the call was already established; there is no
provision for
dialling, ringing or the like.