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Dual-Tone Multi-Frequency or DTMF is a common method for instructing a telephone switching system of a phone number to be dialed, or to issue commands to switching systems or related telephony equipment.
The DTMF standard defines the signal(s) generated when you press a key on your telephone keypad. Each keypress simultaneously generates two tones of specific frequencies: one from a high frequency group of tones and one from a low frequency group (so that a voice can't imitate the tones). The tone combinations are as follows:
|Digit||Low frequency||High frequency||Digit||Low frequency||High frequency|
|1||697||1209 Hz||7||852||1209 Hz|
|2||697||1336 Hz||8||852||1336 Hz|
|3||697||1477 Hz||9||852||1477 Hz|
|4||770||1209 Hz||0||941||1336 Hz|
|5||770||1336 Hz||*||941||1209 Hz|
|6||770||1477 Hz||#||941||1477 Hz|
The full DTMF standard actually defines tones for 16 keys, but most telephones only use 12 keys. The A, B, C and D keys were originally intended for use in menu selection, and are used only rarely for internal signaling within telephone networks today.
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