What is ‘EXT’ on a Fax Machine?

LAST UPDATE: February 17th, 2020

Definition

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

It is near a ‘LINE‘ or ‘TEL‘ port, which connects 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 connects 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.

Usage

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

A phone or answering machine can attach to the EXT port. It functions the same as if 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. Connect it 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.

You can use an attached extension phone 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 stops the transmission.

Similar Terms

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

Frequently Asked Questions

What is the difference between an “EXT” and a “LINE” port?

The “LINE” port is to connect the fax machine to the landline phone jack. The “EXT” port is to connect an extension telephone to the fax machine.

What is the difference between a “TEL” and an “EXT” port?

EXT” is always for an extension phone. “TEL” can be either to connect the fax machine to a phone jack or for an extension phone.
A “TEL” port on its own connects to a landline phone line.
A “TEL” port with an “EXT” port would connect the “TEL” port to a landline phone line and the “EXT” port to an extension telephone.
A “TEL” port with a “LINE” port would connect the “LINE” port to a landline phone line and the “TEL” port to an extension telephone.

What is the difference between an extension phone and a regular phone?

An extension phone is a regular phone that connects through the fax machine (or fax modem). It is a way to have both a fax machine and phone on the same phone line.

How can I test my fax machine to make sure it is set up correctly?

See our pages on “How to Use a Fax Machine” and “How to test a fax machine.”

Sources and more resources