DTMF

Dual-Tone Multi-Frequency or DTMF is a common method for instructing a telephone switching system of a phone number to be dialed, or to issue commands to switching systems or related telephony equipment.

The DTMF standard defines the signal(s) generated when you press a key on your telephone keypad. Each keypress simultaneously generates two tones of specific frequencies: one from a high frequency group of tones and one from a low frequency group (so that a voice can't imitate the tones). The tone combinations are as follows:

The full DTMF standard actually defines tones for 16 keys, but most telephones only use 12 keys. The A, B, C and D keys were originally intended for use in menu selection, and are used only rarely for internal signaling within telephone networks today.