What is “EXT” on a Fax Machine?

LAST UPDATE: June 5th, 2019


‘EXT’ on a fax machine is a port to plug in an extension telephone.

It is found near a ‘LINE‘ or ‘TEL‘ port which is used to connect the fax machine to the phone jack.

Image of a fax machine connected to: 1 - an extension telephone (marked "EXT" or "TEL"), 2 - a landline phone jack (marked "EXT"), and 3 - a telephone handset (marked "TEL")
The “LINE” port is connected to a landline phone line. The “EXT” and “TEL” ports connect to an extension telephone. If there are three ports, the “TEL” port connects to the fax machine’s telephone handset.


Fax machines and fax modems have an ‘EXT’ port to attach an extra phone or answering machine to the same line (and phone jack).

When a phone or answering machine is attached to the EXT port, it will function the same as if it was plugged into the same phone jack as the fax machine.

To use the EXT port:

  1. Make sure your fax machine is set up and working. It will need to be connected to a landline phone line.
  2. Using a phone cord, connect the extension landline phone to the EXT port on the fax machine.
  3. Use the extension phone as you would a regular phone.

An attached extension phone can also be used to check if the phone line is connected and has a dial tone.

Be aware that answering the phone handset when a fax is communicating will stop the transmission.

Similar Terms

Sometimes the “EXT” port is labeled “TEL.” It is found near the “LINE” port.

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