Configuring DTMF signaling
Dual tone multifrequency (DTMF) signaling uses a mixture of a high frequency tone and a lower frequency tone to represent a key on a keypad. Each column of keys is represented by a high frequency tone and each row of keys is represented by a low frequency tone. For example, as shown in Figure 7, the digit 1 is represented by the combination of a pure 697 Hz signal and a pure 1209 Hz signal. These DTMF tones offer good immunity to interference.
Figure 7: DTMF keypad frequencies
A DTMF tone must last a minimum of 45 milliseconds. A minimum interval of 23 milliseconds is required between two DTMF tones to make sure DTMF tones are recognizable. Such requirements are roughly the same in all countries. For more information, see the ITU-T Recommendation Q.24.