I was unable to complete the transfer to the Asterisk Vicidial provider. We performed several tests from the Vicidial side and enabled all firewall rules to allow access to Vapi; however, it’s still not possible to receive requests from Vapi to Vicidial. The call is successfully received in Vapi from the Vicidial provider, but when it tries to transfer the call to a human assistant, it doesn’t work. For some reason, Vapi doesn’t make the request to Vicidial. My theory is that the assistant cannot reach the SIP Trunk configuration and therefore doesn’t initiate the transfer or send the request to Vicidial. Please, help me with this issue.

Thank you for the details. To help diagnose the issue, could you confirm if SIP REFER is enabled on your SIP trunk configuration in Vapi and with your Vicidial provider? This is required for call transfers to work as expected. 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) - [Vapi SIP Trunk documentation](https://docs.vapi.ai/advanced/sip/sip-trunk)

What does "SIP REFER is enabled on your SIP trunk configuration in Vapi" means? and how can it be configured?

Hi GeraDeluxer, It seems you might be experiencing issues with call transfers. To troubleshoot dropped call transfers, here's a troubleshooting workflow for your SIP transfer issue: 1. First, verify the transfer initiation:

curl -X GET "https://api.vapi.ai/call/{call_id}" \  -H "Authorization: Bearer YOUR_API_KEY"

Check if

endedReason

shows "assistant-forwarded-call" 2. For SIP-specific issues: - Check the PCAP file from the call logs (available in the API response) - Use Wireshark to filter for

sip.Method == "REFER"

- Look for: - REFER packet presence (confirms Vapi sent transfer request) - 202 Accepted response (confirms provider accepted) - Error responses (4xx, 5xx indicate rejection) 3. Configuration checks: - Ensure

phoneCallProviderBypassEnabled

is set to

false

- Remove

"phone-call-control"

from

serverMessages

if present - Verify your SIP trunk configuration matches endpoint setup 4. Known limitations: - PSTN-to-SIP transfers are not supported - Cross-provider transfers may have compatibility issues If SIP REFER packets are missing in PCAP, it means Vapi didn't send the transfer request to the SIP provider. This aligns with your theory about assistant-to-SIP trunk connectivity issues. Let me know if these steps resolve the issue or if you need further assistance.

@Geronimo1975 as far as I recall, vicidial doesn't support SIP Refer natively. Please DM and we can investigate.