Issue

A PBX/IPBX is unable to recognize DTMF (Dual-Tone Multi-Frequency) tones sent during SIP trunking calls. This typically results in failures when users attempt to interact with IVRs or acknowledge calls by entering digits on their phones.

Product

Elastic SIP Trunking 

Environment

Twilio Console

Cause

Most SIP trunking providers, including Twilio, transmit DTMF tones using RFC 2833/4733 with a default RTP payload type of 101. If the PBX/IPBX is configured to use a different payload type (e.g., 96) or does not support RFC 2833/4733, DTMF tones may not be recognized, leading to interoperability issues.

Resolution

1. Verify SIP Trunking Support:
   Ensure the PBX/IPBX is configured to support SIP trunking connections.

2. Configure DTMF Payload Type:
   Update the PBX/IPBX configuration to use RFC 2833/4733 for DTMF relay, with RTP payload type 101. This is the industry standard and required for interoperability with most SIP trunking providers.

3. Engage PBX Vendor if Needed:
   If the PBX/IPBX cannot be configured to use payload type 101, contact the PBX vendor or IT support for guidance or possible firmware updates.

4. Testing:
   After making changes, test DTMF recognition by placing a call and entering digits on the phone. Confirm that the PBX/IPBX now recognizes the tones.

Additional Information 

• The DTMF RTP payload type (101) is fixed on most SIP trunking platforms and cannot be changed.
• RFC 2833/4733 is the global standard for out-of-band DTMF transmission in SIP environments.
• For further technical details, consult your SIP trunking provider’s documentation or request their SIP trunking specifications.