Rules

There are three DTMF transmission methods:

1. Inband (Audio)
2. SIP Info
3. RFC-2833 (via special RTP packets)

Core System DTMF Handling

· The system can relay any DTMF method but only accepts RFC-2833 when acting as the DTMF endpoint (e.g., at an Auto Attendant).

• Recommended RFC-2833 Payload: 101
• Identifying DTMF Method & Payload: Check INVITEs and 200 OK responses in trace logs.

Procedure

Identifying the DTMF Method

To locate the DTMF method in a trace:

1. Look for the "m" line and the following "a" lines.
2. For example, using RFC-2833 payload 101:
   
   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

Here, the highlighted areas indicate the DTMF method as RFC-2833 payload 101.

Here it in the first INVITE of a Call Trace:

Resolving Common DTMF Issues

1. Carrier Not Using RFC-2833 Payload 101: Most carriers use RFC-2833 by default. If not, request the carrier to adjust their signaling to this payload.
2. Phone/ATA Not Using or Accepting RFC-2833 Payload 101:
   1. Review the phone or ATA’s administrative guide for DTMF configuration steps.
   2. Add the updated settings to the overrides and retry.
3. DTMF Fails Even with RFC-2833 Payload 101 Enabled:
   1. Test with an alternate carrier or different handset make/model to determine if the issue is specific to a device or carrier.
   2. If the issue persists, coordinate with support for further assistance.