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.
![](https://i1.sndcdn.com/artworks-000031307455-kv8mrt-t300x300.jpg)
![](https://i1.sndcdn.com/artworks-000031307455-kv8mrt-t300x300.jpg)
Long Distance Flashing Supervision Test with MF Pulsing and Tone Applier (location unknown)
![](https://i1.sndcdn.com/artworks-000079361264-huaj03-t300x300.jpg)
![](https://i1.sndcdn.com/artworks-000079361264-huaj03-t300x300.jpg)
Manhattan, New York NYCMNYAA – 753 Supervision Test (N Carrier)
![](https://i1.sndcdn.com/artworks-000079360213-lmeqdb-t300x300.jpg)
![](https://i1.sndcdn.com/artworks-000079360213-lmeqdb-t300x300.jpg)
Queens, New York NYCQNYAS (Astoria) – 545 Supervision Test (Wire Trunks)
![](https://i1.sndcdn.com/artworks-000079359418-39bi0z-t300x300.jpg)
![](https://i1.sndcdn.com/artworks-000079359418-39bi0z-t300x300.jpg)
Ozone Park, New York NYCQNYOP – 835 SupervisionTest (N Carrier)
![](https://i1.sndcdn.com/artworks-000031296018-xcgbtu-t300x300.jpg)
![](https://i1.sndcdn.com/artworks-000031296018-xcgbtu-t300x300.jpg)
Brooklyn to Manhattan “Panel” Supervision Test – (wonderful “Wire Trunk Repeater” background noise)
![](https://i1.sndcdn.com/artworks-000079359829-xrj4tw-t300x300.jpg)
![](https://i1.sndcdn.com/artworks-000079359829-xrj4tw-t300x300.jpg)
Brooklyn, New York NYCKNYTY (Troy Avenue) – 756 Panel Supervision Test (N Carrier)
![](https://i1.sndcdn.com/artworks-000079361029-8qcc52-t300x300.jpg)
![](https://i1.sndcdn.com/artworks-000079361029-8qcc52-t300x300.jpg)
Brooklyn, New York NYCKNYTY (Troy Avenue) – 778 Crossbar Supervision Test (N Carrier)
![](https://i1.sndcdn.com/artworks-000047214866-8y2wcj-t300x300.jpg)
![](https://i1.sndcdn.com/artworks-000047214866-8y2wcj-t300x300.jpg)
Strange Flashing Supervision – (location unknown)