We’re integrating LiveKit voice agents with our telephony platform (Invoca) and running into an issue where tel: URI parameters in the Refer-To header are being stripped by the LiveKit SIP bridge before they reach our downstream FreeSWITCH server. What we’re doing: Our voice agent calls transfer_participant with a Refer-To value of: tel:+15559876543;x-ai-agent-outcome=transfer%3ATech%20Support We can confirm via our agent logs that the SDK is sending this correctly (transfer_sip_refer_sending event shows the full URI with params). What we see on the other side: When the SIP REFER arrives at our FreeSWITCH, sip_refer_to contains only: tel:+15559876543 The ;x-ai-agent-outcome=transfer%3ATech%20Support parameter is dropped somewhere in the LiveKit SIP infrastructure between the agent SDK and the outbound REFER message. Why we need this: We’re using tel: URI parameters (per RFC 3966) to pass metadata like agent outcome back to our call platform during blind transfers. FreeSWITCH reliably propagates sip_refer_to — the params just need to survive the LiveKit SIP bridge. Environment: LiveKit Cloud, using livekit-agents Python SDK, SIP trunk via UDP to FreeSWITCH. Is this a known limitation? Is there a way to preserve tel: URI parameters through the SIP REFER, or an alternative mechanism we should use? Already posted here, but still no response.

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