Ringing (telephony)

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Further information: Ringtone

A ringing signal operates a bell or other ringtone generator in a telephone, to alert the user to an incoming telephone call.

In landline telephones, bells or ringtones are rung by impressing a 60 to 105-volt RMS 20-Hertz sine wave across the tip and ring conductors of the subscriber line, in series with the (typically) -48VDC loop supply.[1] This signal is produced by a ringing generator at the central office. Some 20th-century multi-party lines used ring frequencies other than 20 Hz for selective ringing of the parties.

When the subscriber line is called, a relay on the subscriber line card connects the ringing generator to the subscriber line. The exchange also sends a ringback tone to the calling party. When the called party answers by taking the telephone handset off the switchhook, the subscriber's telephone draws direct current from the central office battery. This current is sensed by the line card and the ringing relay is de-energized.