DTMF (or dual-tone multi-frequency signaling) is the sound a telephone keypad makes when it is pressed. 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 pair of tones – one lower and the other higher. The two tones can easily be understood by the phone company’s automatic switchboard.
When dialing a phone number (or fax number), the buttons that are pressed tell the switchboard where the call should go.