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.
If the called party's line is in use, the exchange returns a busy signal to the calling party. However, if the called party's line is in use but has call waiting installed, the exchange sends an intermittent audible tone to the called party to indicate an incoming call.