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Appendix
Tone Dial Frequencies
The tone dialing method combines two frequencies for each of the twelve digits found on a touch-tone dial
pad. This method is referred to as dual-tone multi-frequency (DTMF) dialing.
The four horizontal rows on a touch-tone keypad use four low-frequency tones (697, 770, 852, 941 Hz), while
the three vertical columns use three high-frequency tones (1209, 1336, 1477 Hz). The tone frequency
tolerance is ± 0.02%.
For example, the digit 4 is dialed by combining two tone frequencies: 770 Hz from the second row, and 1209
Hz from the first column. In another example, the digit 9 is dialed with tone frequencies 852 Hz and 1477 Hz.
Figure B-2. Standard DTMF frequencies Figure B-3. Extended DTMF frequencies
The extended DTMF characters (A, B, C, D) are the high-end frequencies (1633 Hz) defined on some
telephone sets with a fourth vertical column of buttons. This fourth column provides for extended PBX control
functions. The actual functions that are provided depend on the PBX manufacturer’s implementation and
feature set.