Definition
A telephone line that extends a private branch exchange (PBX) or main switchboard to a location physically separate from the main facility, so that the remote site appears as an internal extension of the parent system. In FAA usage, OPX lines connect outlying air traffic facilities, equipment sites, or remote positions to a parent communication system.
Plain English
A phone line that makes a remote site act like an internal extension of a main office, even though it is in a different building or location.
Context Anchor
Seen in FAA acronym and NOTAM contraction lists, usually in connection with airport, facility, or communications information.
Derivation
‘Off premises’ simply means ‘not on the same property,’ and ‘exchange’ is the old telephone term for a switching system that connects calls. Together it describes a phone connection that extends the switchboard beyond the building it lives in.
Why Pilots Care
If OPX appears in an airport or facility notice, it is referring to communications infrastructure, not an aircraft procedure, route, or flight rule.
Intuition Check
Do not read “exchange” as trading or swapping something. Here, “exchange” means a telephone system that connects calls.
Example Sentence 1
The remote transmitter site was tied back to the control facility through an OPX line.
Example Sentence 2
During the move, the OPX kept customer service lines active while the primary office was relocated.