SIP T.38 Fax Relay – Unexpected RTP Payload Type Negotiation Failure

Voice & Telephony
1

The current deployment requires relaying facsimile transmissions via SIP utilizing T.38 as per RFC 3891. We’ve integrated a third-party fax gateway which negotiates the T.38 offer/answer exchange; however, calls initiated from Genesys Cloud are consistently failing to establish the T.38 session. The SDP inspection reveals an issue with RTP payload type negotiation.

Specifically, the Genesys Cloud SIP stack appears to be advertising payload type 97 as dynamic, yet the gateway requires it to be statically defined for T.38 support. This discrepancy leads to the gateway rejecting the INVITE, resulting in a 488 Not Acceptable Here error. The behavior is consistent across multiple outbound dial plans.

The architect flow use a standard outbound voice call module, and no custom SIP headers have been configured. The SIP settings for the trunk connecting to the fax gateway are set to “Allow Presentation Exchange” and “Require Presentation Exchange”, though altering these settings has not resolved the issue. We’ve confirmed that the gateway itself is functioning correctly with other SIP providers.

A review of the Genesys Cloud documentation does not explicitly address static payload type assignment for T.38. This issue has been logged internally as INC-4471. The relevant portion of the SIP stack debug output from the Genesys Cloud side is as follows:

sdp: a=rtpmap:97 PCMU/8000
sdp: a=fmtp:97 mode=1016

The gateway is expecting to see a static mapping-a=rtpmap:97 PCMU/8000-but the Genesys Cloud stack is presenting it as a dynamic allocation. Is there a mechanism to enforce static RTP payload type assignment within Genesys Cloud to enable proper T.38 negotiation?

2

How do I set up the entire SIP trunk, media negotiation, and fax relay pipeline without breaking the state? I’m overwhelmed trying to learn everything at once.

The trunk config needs RTP payload type 97 explicitly defined as static - not dynamic. Something like:

{
 "type": "SIPTrunk",
 "mediaNegotiation": {
 "payloadTypes": [
 {"payloadType": 97, "type": "static"}
 ]
 }
}

ymmv.

3

That payload type fix is right - we’ve seen this before. The SIP trunk config needs that payloadType set to static. Fwiw, double-check the media negotiation settings on the SIP profile itself - sometimes it overrides the trunk settings. It’s in Platform > SIP > SIP Profiles > [your profile] > Media Negotiation. Worth a shot.

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