Dual-tone multi-frequency (DTMF)
DTMF is the signalling method that sends each key pressed on a phone keypad as a pair of tones; it is what IVR menus rely on.
Dual-tone multi-frequency (DTMF) signalling sends each digit on a phone keypad, plus the star and hash keys, as a combination of two tones at different frequencies. Every IVR menu that says "press 1 for sales" relies on it. When a caller presses a key, the system decodes the two tones and works out which key it was. Even now that voice agents understand natural speech, DTMF stays useful wherever precision matters.
How it works
On traditional lines the tones travel inside the audio. In VoIP calls, audio compression can distort them, so key presses are usually sent as separate events. The common standard for this is the IETF's RFC 4733, "RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals", published in December 2006, which replaced the older RFC 2833. Some systems can also carry key presses in SIP messages.
Why it matters
Speech recognition can stumble over long strings of digits, so asking callers to key in an order number or ID number can be more reliable. Some callers would also rather not say sensitive details out loud. A well-designed AI voice agent decides deliberately which details callers should say and which they should type.
In Speakline
In Speakline you build IVR menus with keypad options in the call flow, and the browser-based agent desk has a keypad too. When callbacks are on for a queue, a waiting caller can press 1 to be called back without losing their place (see callback). See Call center for details.