What is DTMF and how does it work?
DTMF (Dual-Tone Multi-Frequency) is a method of transmitting information from a telephone by pressing digits or characters on the keypad. Each key generates a combination of two audio tones at different frequencies, hence the name dual-tone. The receiving system recognises these tones and interprets them as a specific digit or symbol.
DTMF was originally developed for analogue telephone networks and replaced the slower pulse dialling. Today it is used primarily by IVR systems (voice automated menus), self-service systems, and various telephony gateways.
Where is DTMF used in practice?
Most corporate telephone systems use DTMF, even though users are usually unaware of it.
- IVR menu - For the sales department press 1, for technical support press 2. This is a classic example of processing DTMF inputs.
- Confirmation of switching over - the system waits for a key press before transferring the call
- Telebanking - providing your account number or PIN code over the phone
- Voice mailboxes - navigation in the menu for recording and playing back messages
- Telephone authentication - one-time code entered via the phone keypad
DTMF and VoIP PBXs
In a VoIP (internet telephony) environment, DTMF transmission is not as straightforward as in analogue networks. Audio tones can be distorted or lost during voice compression (by the codec). Therefore, VoIP systems transmit DTMF separately from the voice signal, most commonly using RFC 2833 or via SIP INFO messages. An incorrectly configured transmission method will cause the IVR or another system to fail to respond to pressed keys.
If you encounter issues where the IVR does not recognise entered digits, the first thing to check is the DTMF method setting on the SIP trunk and in the PBX. It must be identical on both sides. On VirtualniUstredna.cz, the DTMF method is preconfigured to work with standard operators without further intervention.
Frequently Asked Questions
Why does my IVR not respond to button presses?
The most common cause is a mismatch in DTMF method settings between the SIP trunk and the PBX. Verify that both sides use the same transmission method: typically RFC 2833 or SIP INFO. The settings must not differ.
Does DTMF work over a mobile network?
Yes, mobile networks support DTMF standards by default. Issues may arise when routing through multiple networks or when using a VoIP application (softphone) with an incorrectly configured codec.