Even the ringtone on POTS is 90VAC - used to excite the coils for the big ring bells you see on the top of old phones.
Phone technology is pretty crazy in general - the ringing, dial, and busy tones used to be generated by a magneto attached to a crank and later by racks of DC motors attached to generator sets in phone company offices.
I think some ancient phone systems even used the AC frequency of the ring tone to ring different phones on a shared party / conference line.
> Later, independent systems applied multiple ringing frequencies for fully selective ringing. (The Bell System eschewed frequency selective ringing.) The ringers in party-line phones were tuned to distinguish several different ringing signals so that only the desired party's phone would actually ring. In this arrangement the only inconvenience of a party line was occasionally finding the line in use (by hearing talking) when one picked up the phone to make a call. If one of the parties used the phone heavily, then the inconvenience for the others was more than occasional, as depicted in the 1959 comedy film Pillow Talk.
I was going to say "not that ancient!" because I remember using a shared party line but that was in the 1980s, so pretty old I guess.
Phone technology is pretty crazy in general - the ringing, dial, and busy tones used to be generated by a magneto attached to a crank and later by racks of DC motors attached to generator sets in phone company offices.
I think some ancient phone systems even used the AC frequency of the ring tone to ring different phones on a shared party / conference line.