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.