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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.) This ring voltage came from various sources.
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 near-by mounted ringer box.
Fixed phones of the late 20th century and later detect this ringing current voltage and trigger a warbling tone electronically.
A telephone rings when the telephone network indicates an incoming call, so that the recipient is alerted of the calling attempt.
For landline telephones typically receive an electrical alternating current signal, called power ringing, generated by the telephone exchange to which the telephone is connected.
AT&T offered seven different gong combinations for the "C" type ringer found in the model 5 landline telephone sets.Following a 1975 FCC ruling which permitted third-party devices to be connected to phone lines, manufacturers began to produce accessory telephone ringers which rang with electronic tones or melodies rather than mechanically.People also made their own ringers which used the chip from a musical greeting card to play a melody on the arrival of a call.In North America, the standard ring cadence is "2-4", or two seconds of ringing followed by four seconds of silence.In Australia and the UK, the standard ring cadence is 400 ms on, 200 ms off, 400 ms on, 2000 ms off.