Dual-tone multi-frequency signaling (DTMF) is used for telecommunication signaling over analog telephone lines in the voice-frequency band between telephone handsets and other communications devices and the switching center. The version of DTMF that is used in push-button telephones for tone dialing is known as Touch-Tone. It was developed by Western Electric and first used by the Bell System in commerce, using that name as a registered trademark. DTMF is standardized by ITU-T Recommendation Q.23. It is also known in the UK as MF4.
Other multi-frequency systems are used for internal signaling within the telephone network.
Introduced by AT&T in 1963, the Touch-Tone system using the telephone keypad gradually replaced the use of rotary dial and has become the industry standard for landline service.
Read more about Dual-tone Multi-frequency Signaling: Multifrequency Signaling, #, *, A, B, C, and D, Binary Data Transfer, Keypad, Special Tone Frequencies
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... Hz 440 Hz Dial tone (Most of Europe) 425 Hz ---- As with other multi-frequency receivers, DTMF was originally decoded by tuned filter banks ...