Glossary · DTMF
What is DTMF?
DTMF (Dual-Tone Multi-Frequency) is the touch-tone signaling system that turns each dial-pad key press into a pair of simultaneous sine-wave tones, so the phone network — or a modern IVR — can recognize which digit you pressed. It is the everyday “beep” you hear when navigating a phone menu, and it remains the universal way callers send input to automated systems across both legacy and VoIP networks.
How DTMF works
The dial pad is a 4×4 grid. Each of the four rows is assigned one low-group frequency, and each of the four columns is assigned one high-group frequency. Pressing a key plays its row tone and its column tone at the same time — hence “dual-tone.” The receiver runs the audio through frequency detection (historically a Goertzel-style filter) and maps the detected pair back to a single digit.
The eight frequencies were chosen so that no frequency is a harmonic or simple multiple of another. This keeps each digit’s two-tone combination distinct from voice and from other digits, which is what makes DTMF reliable to decode even over noisy lines.
DTMF frequency table
DTMF is standardized in ITU-T Recommendation Q.23. The full keypad uses the following row (low-group) and column (high-group) frequencies:
| 1209 Hz | 1336 Hz | 1477 Hz | 1633 Hz | |
|---|---|---|---|---|
| 697 Hz | 1 | 2 | 3 | A |
| 770 Hz | 4 | 5 | 6 | B |
| 852 Hz | 7 | 8 | 9 | C |
| 941 Hz | * | 0 | # | D |
The A, B, C, and D keys exist in the standard but were never placed on consumer phones — they originated in military and some specialized signaling uses. Standard phones expose only the twelve keys 0–9, *, and #.
DTMF on VoIP and SIP networks
On the legacy phone network, DTMF tones simply ride inside the audio as real sounds. On VoIP, that approach is fragile: low-bitrate codecs can compress or distort the tones enough that detection fails. To solve this, modern systems send DTMF out-of-band, separate from the voice audio.
The dominant method is RTP telephone-events, defined in RFC 4733 (which obsoleted the earlier RFC 2833). Each digit is transmitted as a dedicated event packet rather than as audio, so the receiver gets the exact digit regardless of codec.
An alternative is SIP INFO, which carries DTMF as signaling messages on the SIP control path rather than in the media stream. RTP telephone-events are generally preferred for interoperability, while in-band audio DTMF is kept only as a fallback for older endpoints.
Where DTMF fits in a business phone system
DTMF is the input layer for almost every automated phone interaction:
- IVR menus: “Press 1 for sales, 2 for support” — the digit is captured via DTMF.
- Auto-attendant: callers key in an extension or directory name.
- Conference bridges: numeric PINs and in-call controls (mute, etc.).
- Account and verification entry: account numbers, card capture in secure payment IVR, survey responses.
- Call-flow controls: transfers and feature codes triggered by
*or#sequences.
Because DTMF is universal across every phone type, it stays the lowest-common-denominator input method even as voice and AI agents handle more of the conversation.
DTMF frequently asked questions
What does DTMF stand for?
DTMF stands for Dual-Tone Multi-Frequency. It is the technical name for the touch-tone signaling system that telephones use when you press a key on the dial pad.
Each key sends two simultaneous sine-wave tones — one from a low-frequency group and one from a high-frequency group — and the receiving system identifies the digit from that unique pair.
How do DTMF tones work?
The dial pad is arranged as a grid of four rows and four columns. Each row maps to one low-frequency tone (697, 770, 852, or 941 Hz) and each column maps to one high-frequency tone (1209, 1336, 1477, or 1633 Hz).
Pressing a key plays the row tone and the column tone together. The four row and four column frequencies were deliberately chosen so none is a harmonic of another, which makes each digit easy to detect reliably.
Is DTMF the same as touch-tone?
Effectively, yes. “Touch-Tone” was AT&T’s trademark for the consumer push-button dialing service, while DTMF (Dual-Tone Multi-Frequency) is the generic engineering term for the same signaling method standardized in ITU-T Recommendation Q.23.
How does DTMF work on VoIP and SIP calls?
On VoIP, sending DTMF as audio inside a compressed voice stream is unreliable because low-bitrate codecs can distort the tones. To avoid this, DTMF is usually sent out-of-band.
The common method is RTP telephone-events defined in RFC 4733 (which obsoleted RFC 2833), where each digit travels as a dedicated event packet alongside the audio. SIP INFO messages are an alternative signaling-path method. Both keep IVR and auto-attendant menu input accurate.
Why does DTMF still matter for business phone systems?
DTMF is how callers interact with automated phone systems. Pressing “1 for sales” in an IVR, entering an extension on an auto-attendant, keying in an account number, or joining a conference with a PIN all rely on DTMF detection. It remains the universal input layer for self-service phone menus.
See how DialPhone fits
DialPhone’s business phone and IVR handle DTMF as out-of-band RTP telephone-events (RFC 4733) by default, so menu input stays accurate even on compressed VoIP call legs — callers reach the right IVR option or extension reliably, with in-band tones kept only as a fallback for legacy endpoints.