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. 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.
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