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.