As FSA, choose System Log from the Admin pulldown menu and check for faxes from the number in question. If they are being automatically routed to a user, there will be a message such as the following:
Info: Move fax 'xyz' to '/var/spool/fax/inbox/joe_user/xyz'
The message will appear two or three lines below the line stating that a fax was received from the number in question.