SIP Trunk 408 Request Timeout on Genesys Cloud Inbound Legs

Can anyone clarify why inbound calls via our primary SIP trunk are returning 408 Request Timeout immediately after the INVITE handshake? The Genesys Cloud side logs show the media session starting, but the ServiceNow webhook for call initiation never fires because the transaction fails before the conversation object is fully established. We are on the eu-west-1 region and the SIP trunk configuration matches the standard RFC 3261 requirements.