When the machine is in Fax/Tel mode, it will use the F/T Ring Time (pseudo/double-ringing) to alert you to pick up a voice call.

If you are at an extension phone, you'll need to lift the handset during the F/T Ring Time and then press # 5 1 between the pseudo/double rings. If no one is on the line, or if someone wants to send you a fax, send the call back to the machine by pressing  5 1.