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.

Lift the external telephone’s handset, and then press Hook to answer.

If you are at an extension telephone, you will need to lift the handset during the F/T Ring Time and then press #51 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 51.