It’s very similar to the sound that dialing a phone (touch tone) makes, but higher pitched and pulsed four times a second. It was VERY LOUD, enough to cause pa

It’s very similar to the sound that dialing a phone (touch tone) makes, but higher pitched and pulsed four times a second. It was VERY LOUD, enough to cause pain if you put it up to your ear. The idea was to help let you know that your phone fell off the receiver even if you weren’t nearby. Edit: the reason this matters is that call waiting was not a standard feature. So if your phone was off the hook, it would not be able to ring and receive a call, the person calling you would just get a “busy signal” which is a lower pitched two-tone beep that is on and off every second. You still hear that from time to time in modern phone systems when they are overloaded.