Dual-Tone, Multiple Frequency (DTMF) tones for touch-tone phones. When a keypad button is depressed, a dual-tone signal is transmitted according to the table below. For example, if the "5" is depressed, then both a 770 Hz tone and a 1336 Hz tone are sent simultaneously. What you actually hear are the two original tones at 770 and 1336 Hz, as well as the sum (2106 Hz) and the difference (556 Hz) frequencies. Fundamental tones are chosen such that these sum and difference frequencies never overlap.
The frequencies were initially designed with a ratio of 21/19, which is slightly less than a whole tone. The frequencies may not vary more than ±1.8% from their nominal frequency, or the switching center will ignore the signal. The high frequencies may be the same volume or louder as the low frequencies when sent across the line. The loudness difference between the high and low frequencies can be as large as 3 decibels (dB) and is referred to as "twist." The minimum duration of the tone should be at least 70 msec, although in some countries and applications DTMF receivers must be able to reliably detect DTMF tones as short as 45ms. On the receiving end, the tones are discriminated and decoded.
Here is a standard DTMF touch tone keypad: