DTMF
DTMF (dual-tone multi-frequency) is the signaling system behind touch-tone dialing. Each key on a phone keypad sends a distinct pair of audio tones that equipment on the line can recognize.
Also called: touch-tone, dual-tone multi-frequency, keypad tones
Every keypad button plays two tones at once, one from a low-frequency group and one from a high-frequency group, giving sixteen combinations for 0–9, * and # plus four rarely used letters. The tones travel in the voice channel, so anything that can hear the call can decode them.
DTMF is how IVR menus read "press 1", how callers enter account numbers, and how some systems accept a PIN. It still matters in a voice-AI world: an agent that takes spoken input usually needs a DTMF fallback for callers who cannot or would rather not speak, and on VoIP the tones may be carried out-of-band rather than as audio.