What is “LINE” on a Fax Machine? Fax Authority >> Glossary >> What is “LINE” on a Fax Machine? LAST UPDATE: April 23rd, 2019 Definition A ‘LINE’ port on a fax is where the telephone line connects. It is found either on its own or near an ‘EXT‘ port (used to connect a phone extension). Usage Fax machines and fax modems have a ‘LINE’ port to connect the fax to a landline phone jack.