How to troubleshoot SIP 488 Not Acceptable Here

⌚ About 2 min read

Compare SDP offers, codecs, and media settings when a trunk or endpoint rejects a call.

Before you start

Work on a copy or a controlled test when the change can affect production. Keep timestamps, screenshots and the previous configuration so the result can be compared.

Step by step

Capture SIP signaling around the INVITE and 488 response.
Compare offered codecs with those allowed on the trunk or endpoint.
Check SRTP, RTP, DTMF, and required SDP attributes.
Review any SDP transformations performed by an SBC or NAT device.
Retest with a minimal offer and document the actual incompatible parameter.

Validation

Repeat the original test after the change and confirm that the expected service works without creating a new regression. Document the final state.

Technical deep dive

SIP 488: rejection usually concerns the proposed media session

Technical checkpoints

488 Not Acceptable Here is often returned when the INVITE is understood but the SDP offer cannot be accepted.
Compare m=audio, payload/codec list, rtpmap, fmtp, SRTP and ptime between the offer and trunk/endpoint capabilities.
An SBC or NAT can modify SDP; capturing on both sides shows whether incompatibility is introduced in transit.

Minimal SDP

Identify the first common codec and retest without unnecessary media options.

m=audio 40000 RTP/AVP 0 8 101
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000

Topic-specific pitfalls

Adding random codecs can move the problem and complicate negotiation.
A 488 is not a one-way RTP problem when the call is not even established.

How to validate

The INVITE contains a compatible offer and the call is accepted without 488.
Two-way RTP and DTMF are then verified after call establishment.