Definition
Dedicated, hard-wired voice communication circuits that connect ATC facilities, sectors, and controllers to one another and to other agencies, separate from the air-to-ground radio frequencies used to talk to pilots.
Plain English
Private phone lines that controllers use to talk to each other and to other facilities. They are not the radios that pilots hear.
Context Anchor
Seen in air traffic control and radio outage procedures, including cases where a center frequency is not working normally.
Derivation
From 'inter-' (between) and 'phone' (voice/sound). It originally referred to internal telephone systems within ships, aircraft, and buildings — voice links between people inside the same operation rather than calls to the outside world.
Why Pilots Care
When a pilot loses radio contact, controllers use interphone lines behind the scenes to coordinate with adjacent sectors and facilities. Knowing this exists helps explain how ATC can still track and protect a NORDO aircraft even when they cannot speak to the pilot directly.
Analogy
Think of interphone lines like a private office phone system for air traffic controllers. The pilot may not be on that call, but the controllers can use it to coordinate what happens next.
Intuition Check
Do not read “lines” as written lines on a page or painted lines on pavement. Here it means communication connections used for voice calls between air traffic control positions or facilities.
Example Sentence 1
When the sector frequency went out, the controller used the interphone lines to ask the next facility to relay instructions to the aircraft.
Example Sentence 2
Clearances were relayed over interphone lines until the primary radio frequency was restored.