It's not that difficult.
Set up an extra ILG/OLG for the fax...
First go to ISDN PRI, and change the incoming and outgoing line groups to '0'.
Then go to the 'Call by Call', and create different groups... your first group might be your regular PRI with 23 channels on your original ILG/OLG.
Then your second one would be on the newly created ILG/OLG for your fax, and would list only 1 "max calls".
Make sure you create another LCR group just for the fax, and send it out the new OLG... and in your DID table, make sure that your fax number is on the new ILG.
That's off the top of my head, I'm sure someone will jump in and correct me if I've made any mistakes!
This same procedure is also how you'd set up to Caller ID the lines differently (Trunks/Calling Number) via OLG groups...