Ringing signal
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A ringing signal is a telephony signal that tells the user that there is an incoming call. This is created by sending an alternating current signal of about 100 volts into the line, where a high-impedance electromagnet is triggered to ring a bell on the phone. Today this signal is transmitted digitally for most of the journey, converted into an alternating current only if the line is not digital end-to-end.
Ringing is said to be "tripped" when the impedance of the line reduces to about 600 ohms when the telephone handset is lifted off the switch-hook. This causes the telephone call to be answered, and the telephone exchange immediately removes the ringing signal from the line and connects the call.
Early research showed that people would wait until the phone stopped ringing before picking it up. Breaks were introduced into the signal to avoid this problem, resulting in the common ring-ring-ring pattern used today. In early party line systems this pattern was a morse code letter indicating who should pick up the phone, but today, with individual lines, the only surviving patterns are a single ring (in the USA) and double-ring (in the UK), originally morse code T and M respectively.
However, party line ringing is making a comeback in some small office and home office situations allowing facsimile machines and telephones to share the same line but have different telephone numbers.
Caller ID signals are sent during the silent interval between the first and second bursts of the ringing signals.