DTMF Signalling - Dual Tone Multi Frequency
DTMF is the signalling method used for dialing by most modern telephones. A pair of frequencies, one from the "low-group" and one from the "high-group" is assigned to each of 12 dial-buttons on the telephone. The additional four DTMF tones A, B, C & D are seldom used. DTMF signalling is also sometimes referred to as Tone Dialing, VF Signalling, MF Dialing or Touch Tone. The "#" symbol is also known by several different names including Crosshatch, Hatch, Pound and Octothorpe.
DTMF Frequency Matrix High-group Frequencies 1209 Hz 1336 Hz 1477 Hz 1633 Hz Low-group Frequencies 697 Hz 1 2 3 A 770 Hz 4 5 6 B 852 Hz 7 8 9 C 941 Hz * 0 # D