What makes a telephone ring?

With standard landlines and the old style mechanical telephones, once the number is dialed the switch applies ringing generator of 90-100 vac at 20 Hz (Ring Voltage) and this causes the ringer in the phone to operate, or 'ring'. After the call is answered, or disconnected, the ringing voltage is removed from the line. The same principal applies to electronic sets, however they do not have mechanical ringers