DTMF Method — VoIP DTMF Transport Detection
Identify which DTMF transport method was negotiated for each call — RFC 4733 RTP telephone-event, SIP INFO, or both. Critical for IVR interoperability testing.
DTMF Method
| Property | Value |
|---|---|
| Key | dtmf_method |
| Unit | String (rtp, info, both) |
| Type | Per-call value |
| Direction | Both |
| RFC | RFC 4733, RFC 2976 |
What It Measures
DTMF Method reports which transport mechanism was used for DTMF digit delivery during the call. The three possible values are:
| Value | Meaning |
|---|---|
rtp | DTMF digits carried as RFC 4733 RTP telephone-event packets in the media stream |
info | DTMF digits carried as SIP INFO messages in the signaling path |
both | Both methods were used during the call (typically during interworking) |
Why It Matters
DTMF transport method is one of the most common sources of IVR failures in multi-vendor SIP deployments. If the sender uses RFC 4733 but the receiver expects SIP INFO (or vice versa), digits are silently dropped — the call connects fine, media flows, but DTMF never reaches the application layer.
This metric eliminates the guesswork. When DTMF digits are sent but not received, checking the method on both sides immediately reveals whether the transport mismatch is the cause.
Common Scenarios
- PBX to SIP trunk: PBX sends RFC 4733, trunk provider expects SIP INFO. Digits never arrive at the IVR.
- SBC interworking: SBC converts between methods. If misconfigured, it may strip DTMF events entirely.
- WebRTC gateway: Browser sends RFC 4733 exclusively. Gateway must forward to SIP INFO if the downstream system requires it.
How to Fix It
- Align DTMF methods across endpoints. Ensure both caller and callee use the same DTMF transport.
- Configure SBC interworking. If your infrastructure requires DTMF method conversion, verify SBC rules explicitly handle the translation.
- Check SDP negotiation. Look for
telephone-eventin the SDP media description. If absent, the endpoint does not support RFC 4733. - Test both methods. Use CallMeter to run the same scenario twice — once with RFC 4733, once with SIP INFO — and compare digit delivery rates.
Related Metrics
- DTMF Digits Sent — Total digits transmitted
- DTMF Digits Received — Total digits received
- Final Status Code — SIP response code if call setup affected by DTMF issues