Do you have access to the 5000 database programming screens? If so, go to SYSTEM/TRUNK RELATED INFO/CALL ROUTING TABLE, find the table you're using (don't know what it will be named) and see if the fax number is in that table. I'm assuming you're using DIDs...if no tables exist we need to go a different route.
Also go to SYSTEM/DEVICES AND FEATURE CODES/PHONES and find your analog ports...these will USUALLY appear at the top of the list and will say SINGLE LINE. Do any of them say fax?
Do you have a physical fax machine, and is it connected to the 5000 in any way?