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A telephone rings when the telephone network indicates an incoming call, so that the recipient is alerted of the call attempt. Landline telephones typically receive an electric alternating current signal, called power ringing or ringing signal, generated by the telephone exchange to which the telephone is connected. The ringing current originally operated an electric bell. For mobile phones, the network sends a message to the recipient's device, which may activate a sound, or a visual or vibrating indication.

On a POTS interface, this signal is created by superimposing ringing voltage atop the −48 VDC already on the line. This is done at the Central Office, or a neighborhood multiplexer called a "SLC" for Subscriber Line Carrier. (SLC is a trademark of Alcatel-Lucent, but is often used generically.) Telephones with electromagnetic ringers are still in widespread use. The ringing signal in North America is normally specified at ca. 90 volts AC with a frequency of 20 hertz. In Europe it is around 60–90 VAC with a frequency of 25 Hz. Some non-Bell Company system party lines in the US used multiple frequencies for selective ringing. Ringing voltage is produced by various sources. Large central offices used motor-driven generator sets for both ringing and other signals such as dial tone and busy signals. In smaller offices, special sub-cycle[1] magnetic oscillators were used. Typically, solid-state oscillators have replaced them. Originally this voltage was used to trigger an electromagnet to ring a bell installed inside the telephone, or in a nearby mounted ringer box. Fixed phones of the late 20th century and later detect this ringing current voltage and trigger a warbling tone electronically. Mobile phones have been fully digital since the early 1990s second-generation ("2G") devices, hence are signaled to ring as part of the protocol they use to communicate with the cell base stations.

While the sound produced is still called a "ring",[2] more-recently manufactured telephones electronically produce a warbling, chirping, or other sound. Variation of the ring signal can be used to indicate characteristics of incoming calls. For example, ringing bursts with a shorter interval between them might be used to signal a call from a given number.

In POTS switching systems, 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 signals that the telephone call has been answered, and the telephone exchange immediately removes the ringing signal from the line and connects the call. This is the source of the name of the problem called "ring-trip" or "pre-trip", which occurs when the ringing signal on the line encounters excessively low resistance between the conductors, which trips the ring before the subscriber's actual telephone has a chance to ring (for more than a very short time); this is common with wet connections and improperly installed lines.

The ringing pattern is known as ring cadence. This only applies to POTS fixed phones, where the high voltage ring signal is switched on and off to create the ringing pattern. In North America, the standard ring cadence is "2-4", or two seconds of ringing followed by four seconds of silence (33% Duty Cycle). In Australia and the UK, the standard ring cadence is 400 ms on, 200 ms off, 400 ms on, 2000 ms off. These patterns may vary from region to region, and other patterns are used in different countries around the world. Some central offices offer distinctive ring to identify which of multiple numbers on the same line is being called, a pattern once widely used on party line (telephony).

In many systems, including North America Bellcore standards, Caller ID signals are sent during the silent interval between the first and second bursts of the ringing signals.

The caller is informed about the progress of the call by the audible ringing signal, often called ringback tone. Power ringing and audible ringing are not synchronized.

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