Why DTMF tones may not register on the SIP Device Engine

This article describes and provides background information about why DTMF tones may not be recognized on the xMatters SIP Device Engine.

SIP DTMF Backgrounder

By definition, Dual-tone multi-frequency (DTMF) signaling is used for telephone signaling over the line in 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.

There are three common ways of sending DTMF on SIP calls:

  • SIP INFO packets
  • As specially marked events in the RTP stream (for details, see RFC2833)
  • Inband as normal audio tones in the RTP stream with no special coding or markers (inband means that DTMF is transmitted within the audio of the phone conversation; i.e., it is audible to the conversation partners. As a result, only uncompressed codecs such as g711 alaw or ulaw can carry inband DTMF reliably).

RFC2833 is an out-of-band method that carries dual-tone multifrequency (DTMF) signaling, other tone signals and telephony events in RTP packets. Separate RTP payload formats are desirable since low-rate voice codecs cannot be guaranteed to reproduce these tone signals accurately enough for automatic recognition. Defining separate payload formats also permits higher redundancy while maintaining a low bit rate.

xMatters supports only RFC2833.

Issue and Symptom

When interacting with xMatters for processing notifications through a phone system either by receiving a call-out or calling in, the end user will be prompted to enter digits for authentication, menu navigation, performing actions, etc. If DTMF detection is not working properly, the system will continue to prompt the user several times before the call is ended. To verify that this is the actual issue, check the xMatters log files, as described in the following sections.

xMatters logging

The xMatters logs must be enabled for Detailed logging before they contain the necessary information to be useful. Check the scripting log file for messages related to DTMF digits that were received after each prompt. For example:

2009-11-18 16:16:40,101 [sipEngineManager.5] DEBUG - - Executing phone API: GetDigits.

2009-11-18 16:16:41,188 [alarmpoint_rtp_read_thread_464] DEBUG - - Get DTMF digit 11 for call 79cc6d30-ec1d-4a71-b470-a22cc4c4ffac

SIP Configuration

The only SIP configuration that affects this is the DTMF Code (101) and the Encoding (ulaw/alaw) in the SIP Device engine settings. Check the following areas and use the associated questions to troubleshoot this issue:

1. SIP extension configuration:

Q. What encodings are allowed?

A: If it allows GSM encoding, remove it; only two encodings should be allowed: alaw or ulaw.

Q. What is the DTMF mode?

A: Ensure that it is set to RFC 2833.

2. Trunk configuration:

Q. What type of trunk is it?

Q. What encodings are allowed?

A: If it allows GSM encoding, remove it. Most providers prefer GSM because it has a higher compression encoding; however, it may cause poor inband DTMF detection.

Q. What is the DTMF mode?

A: Ensure that it is set to RFC 2833.

Q. Is your SIP gateway translating DTMF between extensions and trunks?

A: If the extension and trunk have different DTMF modes or encodings it may be translating.

3. Provider configuration:

Q. What encodings are expected?

A: Ensure they support the configured encodings.

Q. What DTMF modes are expected?

A: Does the provider support RFC 2833, or does it prefer inband?

SIP Audio Logging

Configure xMatters to record the audio from the RTP stream between the SIP Device Engine and the SIP gateway.


Wireshark (available at http://www.wireshark.org/ ) is a network protocol analyzer. It can be configured to capture all network traffic on a particular server. This traffic can be analyzed for SIP related and RTP audio streams. These captures can be saved and sent to xMatters support for analysis.

xMatters Reference

JDN-1327 Originally created by Don Clark

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