If the called party's line is not in use, the exchange sends an intermittent ringing signal (about 90 volts alternating current (AC) in North America and UK and 60 volts in Germany) to alert the called party to an incoming call.
The exchange sends an intermittent ringing signal (about 90 volts alternating current (AC) in North America and UK). This was to alert the called party to an incoming call. The exchange sends an intermittent ringing signal (60 volts in Germany). This was to alert the called party to an incoming call. The called party's line is not in use.