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Acronym for Dual-Tone Multi-Frequency. DTMF is the signaling standard for telephone handset push-button key pads. The keypad is a 4x4 matrix of frequencies; each button corresponding to two frequencies indicating a row and a column:

123A
456B
789C
*0#D

row tones 697, 770, 852, 941
col tones 1209, 1336, 1477, 1633