Hi VAPI Support, We’re seeing an issue when a call comes into our assistant through our Kamailio SIP proxy. The inbound call connects correctly, but when the assistant attempts to transfer the call to an external number, the transfer never reaches the destination. For comparison, direct numbers not routed through Kamailio work fine for transfers, but calls coming through Kamailio are not generating a PCAP file for the outbound leg. Could you help us with: Guidance on why transfers through Kamailio may not be reaching the external destination. Whether it’s possible to get SIP traces for both legs of a failed transfer attempt for debugging. Thanks!

Suggestion: Please verify that your Kamailio SIP proxy is configured to support and correctly forward SIP REFER messages, as Vapi relies on SIP REFER for call transfers. If SIP REFER is not enabled or handled by Kamailio, transfers to external numbers will fail. Example Kamailio config snippet to enable REFER handling:

cfg
# In your request_route block
if (is_method("REFER")) {
    t_relay();
    exit;
}

Make sure this logic is present and that REFER is not being blocked or mishandled in your routing logic. Sources: - [Vapi SIP Trunking Telnyx external number call forwarding troubleshooting](https://discord.com/channels/1211482211119796234/1343963959421829172) - [Vapi documentation: call forwarding](https://docs.vapi.ai/call-forwarding)

The problem is there is no request landing on our Kamailio to even check what the error is, And I do not get the PCaP file in the call logs for these calls. I want to confirm if the transfer is even initiated from VAPI or not.

Hi, could you please provide a call id where you experienced this issue? We would like to review the logs to identify the cause.