Glossary · VoIP and call quality
DTMF
In one sentence
DTMF (dual-tone multi-frequency) is the signal a phone sends when you press a key during a call: two tones played together for each key, which an IVR or remote system decodes.
Also known as : DTMF tones · touch-tone · touch-tone dialling · tone dialling
What is DTMF?
When you press a key during a call, the phone plays a short sound made of two tones at once: a low tone set by the key’s row (697, 770, 852 or 941 Hz) and a high tone set by its column (1209, 1336 or 1477 Hz, plus 1633 Hz for the rarely seen A to D keys). This scheme, dual-tone multi-frequency, or touch-tone, replaced rotary pulse dialling decades ago and is still everywhere.
It is how callers move through an IVR (“press 1 for sales”), enter a conference PIN or account number, check voicemail remotely or pay by card over the phone.
How it works
On an analogue line, the tones simply travel inside the audio. Over VoIP it is less straightforward, because codecs that compress speech, such as G.729, distort pure tones. Three methods are in use:
- In-band: tones stay inside the audio. Reliable with G.711, unreliable with compressed codecs.
- RFC 4733 (formerly RFC 2833): the phone sends a “key 5 pressed” event in the RTP stream alongside the voice. This is the most common method on SIP trunks.
- SIP INFO: the keypress travels as a SIP signalling message, outside the audio stream.
Both ends must agree on the method. If they don’t, digits get lost, doubled (“11” instead of “1”) or arrive too late for the system waiting for them.
Example
An online furniture retailer in Glasgow takes card payments over the phone and wants card numbers kept out of its call recordings. It adds a DTMF masking service between its phone system and the carrier: the customer types the card number on the keypad, the service captures the digits and replaces the tones with flat beeps, and the agent only sees whether the payment went through. Recordings no longer contain card data, which narrows the scope of the firm’s PCI DSS assessment.
DTMF at Universal Telecom
Our virtual PBX handles DTMF end to end: IVR menus, access codes and voicemail work without any set-up on your side. If you connect your own PBX, our SIP trunk supports RFC 4733, and we confirm the right settings with you during onboarding.
Frequently asked questions
Why does an IVR ignore my keypresses?
Usually because the two ends disagree on how DTMF is carried: one sends it inside the audio, the other expects RFC 4733 events, or a compressed codec has distorted the tones. Matching the DTMF mode on the trunk fixes most cases.
Is RFC 2833 the same as RFC 4733?
Effectively, yes. RFC 4733 replaced RFC 2833 in 2006 and describes the same idea of sending keypresses as events in the RTP stream. Many PBX settings menus still use the older name.
Can card payments be taken by keypad over the phone?
Yes, and it is a common way to keep card numbers out of call recordings. Specialist services intercept and mask the DTMF tones so agents never hear or see the digits, which helps with PCI DSS compliance.
Related terms
- IVR (interactive voice response)
IVR (interactive voice response) is the automated phone menu that greets callers and routes them based on the keys they press, such as “press 1 for sales, press 2 for support”.
- Codec
A codec is the algorithm that digitises and compresses your voice during a VoIP call and turns it back into sound at the other end. It sets both the audio quality and the bandwidth used.
- SIP
SIP (Session Initiation Protocol) is the standard signalling protocol that sets up, changes and ends VoIP calls. It handles ringing, answering and hanging up; the voice itself travels separately.
- SIP trunk
A SIP trunk is a voice-over-IP connection between a business phone system and a carrier. It replaces ISDN lines and carries calls and numbers over your internet connection.
- VoIP
VoIP (voice over internet protocol) carries phone calls as packets of data over an IP network, such as the internet or an office LAN, instead of over the copper lines of the traditional phone network.
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