Apparently it is a feature Called Subscriber Held (CSH). https://security.stackexchange.com/a/100342/143105 https://security.stackexchange.com/a/100342/143105

Apparently it is a feature Called Subscriber Held (CSH). https://security.stackexchange.com/a/100342/143105 https://security.stackexchange.com/a/100342/143105 TL;DR It was just how analog phone worked, users came to rely on it, digital exchanges reimplemented it (with a timeout)