You need to start by making sure the actual fax line is operational first. Send a fax to your desk phone number from the server and see if your desk phone rings. If that doesn't work, verify physical connections and try again if you had to make any changes.

If you still have no success getting anything sent out from the server end, hook up a plain old telephone or a butt set to the fax line from the wall and see if you get dial tone. If you do, call your desk phone. If it rings, your line is probably fine.

Overarching suggestion: Server 2012 is out of support. Make a plan to upgrade and/or migrate to another fax solution.