What is LINE on a fax? Definition and usage Fax Authority > Glossary > What is LINE on a fax? Definition and usage LAST UPDATE: March 5th, 2018 Definition A ‘LINE’ port on a fax machine or fax modem is where the telephone line connects. There is often an ‘EXT’ port which is used to connect a phone extension. Usage Fax machines and fax modems have a ‘LINE’ port to connect the fax to the phone jack.