# SIP REFER Refer-To URI parameters stripped by LiveKit SIP bridge
**URL:** https://community.livekit.io/t/sip-refer-refer-to-uri-parameters-stripped-by-livekit-sip-bridge/942
**Category:** Telephony
**Tags:** sip-trunking
**Created:** [April 23, 2026, 6:22pm UTC](https://community.livekit.io/t/sip-refer-refer-to-uri-parameters-stripped-by-livekit-sip-bridge/942 "2026-04-23T18:22:16Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Patrick\_Hurley](https://community.livekit.io/u/Patrick_Hurley)
#### Post date: [April 23, 2026, 6:22pm UTC](https://community.livekit.io/t/sip-refer-refer-to-uri-parameters-stripped-by-livekit-sip-bridge/942/1 "2026-04-23T18:22:16Z")
Hi team — 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?
---
### Author:  [@Patrick\_Hurley](https://community.livekit.io/u/Patrick_Hurley)
#### Post date: [April 24, 2026, 11:30am UTC](https://community.livekit.io/t/sip-refer-refer-to-uri-parameters-stripped-by-livekit-sip-bridge/942/2 "2026-04-24T11:30:27Z")
Follow up message. It was suggested that not wrapping transfer\_to in \< \> brackets was the issue. Ran another set of tests with:
```auto
{
'destination': 'Tech Support',
'transfer_to': '',
'phone_number': '+155 59876543',
'participant': 'sip\_+17349915093',
'participant_sid': 'PA_8GejPyYKuDW4',
'event': 'transfer_sip_refer_sending',
'logger': 'voice_agent.agents.tools.transfer_call',
'level': 'info',
'@timestamp': '2026-04-24T02:39:05.020491Z'
}
```
And then the response to our FreeSWITCH was observed using:
```auto
ngrep -qT -W byline -d any 'REFER' 'udp and port 5060'
```
as
```auto
U +12.990509 161.115.178.125:5060 -> 10.203.39.121:5060
REFER sip:mod_sofia@3.86.178.113:5060 SIP/2.0.
Record-Route: .
Record-Route: .
Via: SIP/2.0/UDP 161.115.178.125:5060;branch=z9hG4bK.d4fff2124c3567bc9392e3f0.sipfe.
Via: SIP/2.0/TCP 10.35.164.7:40004;branch=z9hG4bK.d4fff2124c3567bc9392e3f0.sipfe.
Via: SIP/2.0/TCP 161.115.179.25:9000;branch=z9hG4bK.5E2yZps1DRQVdhcp;alias.
Max-Forwards: 69.
Call-ID: 34306a3e-ba2d-123f-699e-12962279f1d9.
Contact: .
Refer-To: .
Allow: INVITE, ACK, CANCEL, BYE, NOTIFY, REFER, MESSAGE, OPTIONS, INFO, SUBSCRIBE.
X-AI-Agent-Outcome: transfer:Tech Support.
Content-Length: 0.
CSeq: 113946155 REFER.
From: ;tag=SCL_VDDyG8mrhaGm.
To: "Patrick" ;tag=N4y26522am16g.
```
Noting specifically:
```auto
Refer-To:
```
Which lost the x-ai-agent-outcome parameter.
Thanks for your time and attention.
---
### Author:  [@Hamdan\_Sayeed](https://community.livekit.io/u/Hamdan_Sayeed)
#### Post date: [April 28, 2026, 11:10pm UTC](https://community.livekit.io/t/sip-refer-refer-to-uri-parameters-stripped-by-livekit-sip-bridge/942/3 "2026-04-28T23:10:48Z")
Hi @Patrick_Hurley, apologies for the delay. Could you expand more on where the `Refer-to` field is getting stripped? If you look at the PCAP for [this](https://cloud.livekit.io/projects/p_2lddta7nrg5/telephony/SCL_VDDyG8mrhaGm/inbound) session as an example, you can see that the `Refer-to` field is populated fully as you provided in the api, ie: `tel:+15559876543;x-ai-agent-outcome=transfer%3ATech%20Support`. If you are indeed seeing URIs getting stripped, any chance that the FreeSWITCH server is the one that is stripping the URI parameters?