In telephony, supervision means detecting whether a phone line or circuit is “on-hook” or “off-hook”.
A line goes off hook to get dial tone and/or originate a call, and also when answering an incoming call. It goes on hook when hanging up to end a call.
Because this was an important function to test, central offices typically had a special number assigned to a supervision test circuit. If you called these numbers, you’d hear the line ring, and then the test circuit would go off-hook and on-hook a few times.
Supervision tests were fun to listen to, and like so much of the old network, sounded slightly different depending on which central office you called.
Long Distance Flashing Supervision Test with MF Pulsing and Tone Applier (location unknown)
Manhattan, New York NYCMNYAA – 753 Supervision Test (N Carrier)
Queens, New York NYCQNYAS (Astoria) – 545 Supervision Test (Wire Trunks)
Ozone Park, New York NYCQNYOP – 835 SupervisionTest (N Carrier)
Brooklyn to Manhattan “Panel” Supervision Test – (wonderful “Wire Trunk Repeater” background noise)
Brooklyn, New York NYCKNYTY (Troy Avenue) – 756 Panel Supervision Test (N Carrier)
Brooklyn, New York NYCKNYTY (Troy Avenue) – 778 Crossbar Supervision Test (N Carrier)
Strange Flashing Supervision – (location unknown)