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(SLC is a trademark of Alcatel-Lucent, but is often used generically.) This ring voltage came from various sources.
Large central offices used motor-driven generator sets for both ringing & other signals such as dial tone and busy signals.
The ringing signal in North America is normally specified at ca. In Europe it is around 60–90 VAC with a frequency of 25 Hz.
Telephones with electromagnetic ringers are still in widespread use.
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).