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. 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.
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. For example, ringing bursts with a shorter interval between them might be used to signal a call from a given number. Variation of the ring signal can be used to indicate characteristics of incoming calls.
While the sound produced is still called a "ring", more-recently manufactured telephones electronically produce a warbling, chirping, or other sound. 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. Fixed phones of the late 20th century and later detect this ringing current voltage and trigger a warbling tone electronically. Originally this voltage was used to trigger an electromagnet to ring a bell installed inside the telephone, or in a nearby mounted ringer box. Typically, solid-state oscillators have replaced them. In smaller offices, special sub-cycle magnetic oscillators were used.
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Large central offices used motor-driven generator sets for both ringing and other signals such as dial tone and busy signals. Ringing voltage is produced by various sources. Some non- Bell Company system party lines in the US used multiple frequencies for selective ringing. In Europe it is around 60–90 VAC with a frequency of 25 Hz. 90 volts AC with a frequency of 20 hertz. The ringing signal in North America is normally specified at ca. (SLC is a trademark of Alcatel-Lucent, but is often used generically.) Telephones with electromagnetic ringers are still in widespread use. This is done at the Central Office, or a neighborhood multiplexer called a "SLC" for Subscriber Line Carrier.
On a POTS interface, this signal is created by superimposing ringing voltage atop the −48 VDC already on the line. For mobile phones, the network sends a message to the recipient's device, which may activate a sound, or a visual or vibrating indication. The ringing current originally operated an electric bell. 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. A telephone rings when the telephone network indicates an incoming call, so that the recipient is alerted of the call attempt.
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