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Glossary · Voice technology

What is DTMF (touch tones)?

  • Also called: touch tone, dual tone multi frequency, keypad tones

Reviewed by Sujan ThapaliyaLast updated

Definition

DTMF is the pair of tones a phone sends when a key is pressed — the sound that lets an IVR know you chose option two. Each key transmits two simultaneous frequencies, which is what makes the signal distinguishable from speech.
The three STIR/SHAKEN attestation levels, which underpin how dtmf (touch tones) behaves in practice: A asserts both that the carrier knows the customer and that the customer may use the calling number, B asserts the customer only, and C asserts neither. A shared number pool can reach B at best, which is why number ownership keeps recurring in these definitions.
Attestation is the backdrop to most caller-ID and deliverability terms, dtmf (touch tones) included.

In practice

How dtmf (touch tones) actually works

On IP calls, tones can be carried in three different ways: inside the audio, as separate signalling events, or in SIP messages. Two systems choosing differently is the usual reason keypresses are ignored by an IVR.

Compression makes in-band tones unreliable. Codecs designed for speech distort the frequencies enough that detection fails, which is why out-of-band signalling became the norm.

DTMF is also a security consideration: keying a card number sends the digits as tones that a recording captures unless the system masks them, which is why payment flows pause recording or suppress the tones.

Tone reliability also degrades on mobile networks in a way that catches out payment flows. Compression and handover can distort in-band tones enough that a sixteen-digit card number arrives with two digits wrong, which reads to the caller as the system being broken and to the business as an abandoned payment.

Worked example

An IVR that ignores keypresses from one carrier and accepts them from another is not broken: the two sides disagree on whether the tones travel inside the audio or as signalling events.

DTMF (touch tones): common questions

Why does an IVR not register my keypresses?
Usually a mismatch in how the tones are carried — one side sending them inside the audio while the other expects signalling events. It is a configuration issue rather than a fault with the keypad.
What does DTMF stand for?
Dual-tone multi-frequency. Each key sends two tones at once, which is how the receiving system tells a keypress from speech or background noise.
Are keypad tones captured in call recordings?
Yes, unless they are deliberately masked or the recording is paused. That matters wherever callers key in card or account numbers, since the tones can be decoded from the audio.

Sources

  1. Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.
  2. 47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
  3. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

See dtmf (touch tones) in the product

An IVR (interactive voice response) system answers inbound calls and routes them using a menu, a spoken request, or data it already holds about the caller. Good IVR gets someone to the right person in one step. Bad IVR is the reason people press 0 repeatedly.

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