DTMF (or dual-tone multi-frequency signaling) is the sound a pressed telephone keypad. This sound is heard by the telephone company, which then uses the information to route the call or use a telephone feature.
DTMF keypads were responsible for replacing the rotary dialers on telephones.
Each key on a telephone keypad emits a pair of tones – one lower and the other higher. The phone company’s automatic switchboard can easily understand the two tones.
When dialing a phone number (or fax number), the pressed buttons tell the switchboard where the call should go.