How do I troubleshoot DTMF Issues?

Concept

DTMF (dual tone multi frequency) is the signal that is generated when a caller presses the keys on an endpoint.  DTMF usually fails due to the wrong DTMF method/value.

Rules

  • There are three standards for DTMF – inband (as audio), via SIP Info, or RFC-2833 (special RTP packets).
  • Core can relay any method but only accepts RFC-2833 when it is the DTMF endpoint (e.g. at an Auto Attendant).
  • The recommended RFC-2833 payload is 101
  • You can identify the DTMF method and payload by reviewing the INVITEs and 200OK in the trace.

Procedure

Identifying DTMF Method

  1. Look for the “m” line and “a” lines that follow. The example below is using RFC-2833 payload 101 as identified by the highlighted areas.
    m=audio 24190 RTP/AVP 18 0 101
    a=rtpmap:0 PCMU/8000
    a=rtpmap:18 G729/8000
    a=fmtp:18 annexb=no
    a=rtpmap:101 telephone-event/8000

Resolving the Problem

  1. Carrier Isn’t Using RFC-2833 Payload 101 – Most carriers do this by default, but if not, ask them to change their signalling.
  2. Phone/ATA Not Using/Accepting RFC-2833 Payload 101 – Check the phone/ATA’s admin guide for directions how to change DTMF settings. Add those settings to the NDP and retry.
  3. DTMF Fails Even with RFC-2833 Payload 101 Enabled – Try an alternate carrier or handset make/model to see if the problem is device or carrier specific. From there work with the appropriate vendor to address.