On original POTS circuits the ring tone was actually the 25Hz signal sent to the phone with the phone's bell coils supplying some of the ringback harmonics along with a ring generator. With a good ear you could estimate how many phone sets were ringing. The audio path was already set-up while the phone rang. If the called party was too near an AM broadcast transmitter you might even hers some of the program between rings. The off-hook condition on the called party just disconnected the ring generator at the CO and started any billing equipment.
Of course this all ended with the last of the Stroger and crossbar offices.