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DTMF

Dual-Tone MultiFrequency; Touch Tone, Tone Dialing
Used for telephone signaling over the line in the voice frequency band to the call switching center. DTMF is an example of a multifrequency shift keying (MFSK) system. Today DTMF is used for most call setup to the telephone exchange, at least in the Western world, and trunk signalling is now done out of band using the SS7 signaling system. The trunk signalling tones were different from the tones known as touch tone with a triangular matrix being used rather than a square matrix.
See also: SS7

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