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.
- 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.
Identifying DTMF Method
- 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
Resolving the Problem
- Carrier Isn’t Using RFC-2833 Payload 101 – Most carriers do this by default, but if not, ask them to change their signalling.
- 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.
- 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.