DTMF
From The RadioReference Wiki
Dual-tone multi-frequency
DTMF signaling is used for voice-frequency signaling over radio channels or telephone lines. In radio signaling, this is commonly used for tone-squelch or fire tone-out type applications. It was developed by the telephone industry and today is still its most common use, known by the trademarked term Touch-Tone.
For history and technical details including the frequencies, see Wikipedia http://en.wikipedia.org/wiki/Dual-tone_multi-frequency