Way back, in the UK, the call didn’t drop if the answering party hung up. The caller got a tone to tell them the other party had disconnected or hung up, but if the answering party picked up (or reconnected) again, the call would continue.

I think they changed it because fraudsters were using it. They’d call someone, claim to be the bank, and tell the person to call their branch as a security measure. The fraudster would then play the disconnected sound until the victim hung up, then they’d play the dial tone, and wait for the victim to pick up and dial their branch number. The fraudster could then act like the brank again, and persuade the victim to give them their secure details, as they were obviously talking to the bank.