it's a tone played through the earpiece speaker, but it is very loud - ie. loud enough you can hear it from another room in your house. https://en.wikipedia.or

it's a tone played through the earpiece speaker, but it is very loud - ie. loud enough you can hear it from another room in your house. https://en.wikipedia.org/wiki/Off-hook_tone#:~:text=In%20the%20United%20Kingdom%2C%20a,referred%20to%20as%20a%20howler https://en.wikipedia.org/wiki/Off-hook_tone#:~:text=In%20the.... I assume phones have to be designed with the earpiece speaker robust enough not to be fried by such a tone, and therefore it must have been around for a long time.