I've always done it as follows with great results (taken from the docs, numbers given are examples obviously):
*******
In the following example, when someone dials 5551234, the system dials 501, which is then rerouted to the fax server:
Attach the fax modem to extension 216
Configure extension 216 with Caller Display Type = DTMFF
Create a Hunt Group set to extension number 500, with extension 216 as a member.
Create a User, eg. Fax501 with an extension of 501 and set Forwarding Unconditional to 500
Create an Incoming Call Route to extension 501 where the Incoming Number is the user's required personal fax number. Repeat for each user's personal fax number.
Incoming Number: 5551234
Destination: 501 Fax 501
********
Of course I would substitute the "Repeat for each users fax number" with the same idea as what IPGuru was expressing.
Incoming number: -41655512
Destination: 501 Fax 501
Peter Sherwood
Morrack Consulting