Telecoms glossary • D
DTMF (Dual-Tone Multi-Frequency)
The tones behind every menu, PIN prompt and automated banking line. On VoIP there is more than one way to send them, and that is where it breaks.
What is DTMF?in VoIP and business telephony
1. DTMF is the tone signalling a telephone keypad uses. Pressing a key sends two audio frequencies at the same time, one from a low group and one from a high group, and the combination identifies the digit. It is what an IVR menu, a voicemail PIN prompt and an automated banking line are listening for.
2. On a traditional line the tones simply travelled with the speech. On VoIP they can travel that way or as separate signalling events, and which method each end is configured for is the difference between a menu that works and one that misses every second press.
How a key press becomes a digit
Every key sits at the intersection of one row frequency and one column frequency. Two tones rather than one is deliberate: no human voice and no music produces that exact pair, so a receiver can tell a deliberate key press from speech.
| Low group | 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 to D column exists in the standard but almost never appears on a handset. The behaviour of a push-button set is specified in ITU-T Recommendation Q.23, approved in 1988 and still in force, which is roughly how long these four row frequencies have been unchanged.
In band or out of band, and why it matters
A VoIP endpoint has three ways to tell the far end which key was pressed. Getting all parties onto the same one is the whole of DTMF troubleshooting.
| Method | How it travels | Reliability |
|---|---|---|
| In band | As real audio tones inside the RTP stream, like any other sound. | Survives on G.711. A compressing codec such as G.729 was designed for speech and can distort the tones badly enough that the far end misses digits. |
| RFC 4733 | As named events in RTP, carrying the digit and its duration rather than the sound of it. | The default on almost every platform, and the setting to use. Immune to codec compression because the digit is never audio. |
| SIP INFO | As SIP messages on the signalling path, separate from the media. | Works, but the timing is looser and support is uneven between platforms. Kept for compatibility with older equipment. |
Most vendor interfaces still label the middle option RFC 2833, which is the earlier specification RFC 4733 formally obsoleted. The label is stale, the setting is the right one to pick.
When DTMF is actually the fault
DTMF problems present as a menu that ignores you, and they are almost always a mismatch rather than a network fault.
- Digits missed entirely
- One end is sending in band while the other expects events, or a compressing codec has been negotiated on a call carrying in-band tones. Symptom: the call audio is clean, the menu simply never responds.
- Digits doubled
- Both methods are enabled at once, so the far end counts the event and hears the tone. Pressing 1 registers as 11 and the menu drops you somewhere unexpected.
- Works internally, fails to the bank
- Your own IVR is forgiving; a banking or payment line is not. If key presses only fail against external automated systems, the mismatch is at the trunk, not on your handsets.
- Not DTMF at all
- If the caller also reports gaps or robotic audio, you are looking at packet loss and the key presses are collateral damage. Fix the line first.
None of these need new hardware. All of them are a setting on the extension, the trunk or both, and a provider who cannot change the DTMF method for you should explain why.
How to report it so it gets fixed once
Tell the provider three specifics: what you are dialling into, whether digits are being missed or repeated, and whether it fails on every call or only on some. A ticket that says the IVR is not working will bounce back asking you to reboot the phone.
If you are running an automated payment or card-capture flow over the phone, raise DTMF handling before you build it. Recording a call that carries card digits as audible tones creates a POPIA problem your call recording policy has to answer for, and suppressing those tones from the recording is a platform feature you have to ask for.
About this entry
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