The Dual Tone Multi Frequency (DTMF) method is a frequency shift keying (FSK) method used in the dialing process in telephone equipment. The dual tone method has replaced the pulse dialling method used in older telephone sets and is used for faster call set-up in communication networks
. The DTMF signal consists of two tones generated by the dial pad and sent to the exchange. The two tones are obtained from eight different tones and are assigned to the rows (1, 4, 7, star) and the columns (1, 2, 3) of the telephone keypad. Using the DTMF method, menu-controlled services (e.g. answering machines, voiceboxes
) can also be controlled directly via the telephone keypad.tone frequency (697 Hz, 770 Hz, 852 Hz and 942 Hz) and one from the higher tone range (1,209 Hz, 1,336 Hz, 1,477 Hz and 1,633 Hz). There are also tactical phones for special applications that have a total of 16 keys, four more than the commonly used one. The additional buttons are for Flash Override (FO), Flash (F), Immediate (I), and Priority (P). There are special frequencies for these keys. The upper frequency is always 1,633 Hz, the others 697 Hz, 770 Hz, 852 Hz and 942 Hz.