ffeat: Implement custom RFC 4733 DTMF decoding with End-bit detection

[usluio]

Summary

This PR implements custom DTMF RTP packet decoding that bypasses the Marker bit requirement
from media-sdk, fixing an issue where DTMF events were silently dropped when SIP providers
do not set the RTP Marker bit on DTMF packets.

Problem

The existing dtmf.DecodeRTP() from media-sdk requires the RTP Marker bit to be set
on incoming DTMF packets as specified in RFC 4733.
However, many SIP providers/gateways do not consistently set the Marker bit, causing DTMF
events to be completely ignored. This results in:

• IVR/PIN input failures
• DTMF forwarding to LiveKit rooms silently failing
• No logging to indicate why DTMF was being dropped

Solution

Custom DTMF Decoding (media_port.go)

• Replaced dtmf.DecodeRTP() with a custom decoder that directly parses the
  RFC 4733 §2.3 payload format
• Uses the End bit (byte 1, bit 7) instead of the Marker bit to determine when a DTMF
  event is complete, preventing duplicate events
• Maps event codes (0-15) to their corresponding DTMF digits (0-9, *, #, a-d)
  as defined in RFC 4733 §7 (Event Code Registry)
• Added debug/info logging for incoming DTMF RTP packets

Improved DTMF Forwarding (inbound.go)

• Added structured logging throughout the DTMF handling pipeline
• Error handling for SendData() — previously errors were silently discarded with _ =
• Log messages for: DTMF received, forwarded successfully, forward failed, buffered for PIN,
  and buffer full conditions

Changes

• pkg/sip/media_port.go — Custom RFC 4733 DTMF payload decoding
• pkg/sip/inbound.go — Enhanced DTMF forwarding with error handling and logging

References

• RFC 4733 — RTP Payload for DTMF Digits, Telephony Tones, and Signals
• RFC 4733 §2.3 — Payload Format
• RFC 4733 §2.3.1 — Marker Bit
• RFC 4733 §7 — Event Code Registry
• RFC 2833 (Obsoleted by RFC 4733)
• livekit/media-sdks — DTMF DecodeRTP

Testing

• Tested with SIP providers that do not set the Marker bit on DTMF packets
• Verified DTMF digits are correctly decoded and forwarded to LiveKit rooms
• Confirmed no duplicate events due to End-bit filtering

[CLAassistant]

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[dennwc]

Please file a PR to the media-sdk and add it as an alternative decoding mode (parameter or a new function) instead.

[usluio]

Can you point this file for this project? @dennwc

[codecov]

Codecov Report

❌ Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.63%. Comparing base (0460b40) to head (aa17506).
⚠️ Report is 219 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/sip/inbound.go	55.55%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #585      +/-   ##
==========================================
- Coverage   65.25%   64.63%   -0.62%     
==========================================
  Files          51       34      -17     
  Lines        6588     6527      -61     
==========================================
- Hits         4299     4219      -80     
+ Misses       1915     1888      -27     
- Partials      374      420      +46     

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[usluio]

We just needed to these logs for the point issue. When decode has an occured we can' t understand or see from logs. Needs that!

[usluio]

Can you point this file for this project? @dennwc