If the phone is accidentally left off the hook (or if there is an electrical problem with the wiring so that it is _reading_ as off the hook), the exchange may

If the phone is accidentally left off the hook (or if there is an electrical problem with the wiring so that it is _reading_ as off the hook), the exchange may drive it with a very loud fast BEEP-BEEP-BEEP-BEEP sound. That's just part of how POTS (plain old telephone service) lines work. Traditionally a user would not want to leave the phone off the hook accidentally, because then that line could not receive calls, and if it was a party line, no one else on the line could receive calls or make calls. If it stays off-hook long enough, eventually this will go silent. See: https://en.wikipedia.org/wiki/Off-hook_tone https://en.wikipedia.org/wiki/Off-hook_tone And: https://en.wikipedia.org/wiki/Permanent_signal https://en.wikipedia.org/wiki/Permanent_signal