What is EXT on a fax? Definition and usage Fax Authority > Glossary > What is EXT on a fax? Definition and usage LAST UPDATE: April 8th, 2018 Definition ‘EXT’ on a fax machine is a port to plug in a phone extension. It is usually found near a ‘LINE‘ port which is used to connect the fax machine to the phone jack. Usage Fax machines have an ‘EXT’ port to attach an extra phone or answering machine to the same line (and phone jack).