A telephone “ring” is the sound generated when there is an incoming telephone call. The term originated from the fact that telephones originally had a ringing mechanism consisting of metal bells and an electromagnetically-driven clapper, producing a ringing sound. The electrical signal powered the electromagnets which would rapidly move and release the clapper, striking the bells.
This electromagnetic bell system is still in widespread use. The ringing signal sent to a customer's telephone is 90 volts AC at a frequency of 20 hertz in North America. In Europe it is around 60-90 volts AC at a frequency of 25 hertz. Some non-Bell system party lines in the US used multiple frequencies (20/30/40 Hz, 22/33/44 Hz, etc.) to allow "selective" ringing.