DTMF (Dual Tone MultiFrequency) A signaling method that employs fixed pairs of frequencies. Most analog touchtone telephone sets are equipped with DTMF keypads in a grid three keys on the vertical axis and four keys on the horizontal axis, although some special purpose sets have a 4 × 4 grid.As the user depresses each key on the pad, the telephone set transmits a paired set of tones, comprising one high frequency tone and one low frequency tone, across a link to a switch that receives and stores them in a register for use in call setup. See Figure D-3 for an illustration of the DTMF grid. Hz 1209 1336 1477 1633 697 1 2 3 A 770 4 5 6 B 852 7 8 9 C 941 * 0 # D |