Recently at a branch office of one of our customers, we needed to find where the fax line terminated in the wire closet so we could install a LAN Modem.  We contacted one of the technichians for the company who installed the phone system.  The technician did not know where the line terminated but said a quick way to find out was to unplug the phone line from the device, so that it will continue to ring, dial the number, and then use your finger to bridge each connection on the punchdown panel.  The one that “jumps up and bites you” is the line you are looking for.  A less manly (but slightly safer) way to do this is to use a screwdriver instead of your finger and listen for the call to sound like it connected as you check each line on the punchdown panel.