DTMF

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Overview

Dual-tone multi-frequency (DTMF) signaling is used for telephone signaling over a line for the voice-frequency band to the call switching center. The version of DTMF used for telephone tone dialing is known by the trademarked term Touch-Tone, and is standardized by ITU-T Recommendation Q.23. Other multi-frequency systems are used for signaling internal to the telephone network. [[1]]

SIP DMTF signaling is done with INFO messages or with RTP messages, the media protocol. This is defined here: http://www.ietf.org/rfc/rfc2833.txt

More information on the DTMF types in Asterisk can be found here: http://www.voip-info.org/wiki/view/Asterisk+sip+dtmfmode

Inband DTMF

  • We have discovered that some SIP providers do not support either rfc2833 or SIP INFO messages (they only send the audio). In this case, the only solution is to use inband DTMF signaling. The only drawback to this solution is that you must use ulaw or alaw for your voice codec. If you try to use another codec, the compression can cause the tones to be unrecognizable.

Inband DTMF


In the diagram above, you can see that the VoiceRD PBX is able to accept all types of incoming DTMF signals from the phone. VoiceRD then sends the DTMF signals out to our SIP provider via inband DTMF; this assumes that the SIP provider is setup as a Device in the VoiceRD system.

  • The DTMF type is on a per-device basis. You can change the DTMF type for each device on the Device Settings page of the VoiceRD web console.
  • IMPORTANT NOTE: You MUST use alaw or ulaw for your codec or inband DTMF will fail. See image for an example:
Setting DTMF and Codecs for our Device
Setting DTMF and Codecs for our Device

Asterisk DTMF and Telasip

  • This is how you can configure VoiceRD to correctly handle DTMF when using Telasip as a SIP provider.

The context you send your SIP user to, must start with the following line: 

  exten => _X.,1,SIPDtmfMode(rfc2833)
  • This will force incoming calls using the Telasip trunk to use the rfc2833 DTMF protocol.
  • Outgoing calls will use inband DTMF (see image and instructions in the section above).
  • This setup provides the best compatibility with older traditional phone systems.
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