Hello, can someone maybe help me out? I’ve got this problem: when there’s an incoming call to the assistant, the transfer call function works perfectly. But when the assistant makes an outbound call, after the agent triggers the transfer call function and delivers the transfer message, the call just ends on the user’s side and the transfer never reaches our target number. What could be the issue? I’m using Telnyx for SIP. https://cdn.discordapp.com/attachments/1420812860090617887/1420812860371767449/telegram-cloud-photo-size-2-5382024695894899288-y.jpg?ex=694234d7&is=6940e357&hm=9a33e0463e2c3aa3ec1f4df58c56db2ec1314136a57e28c854a8c24e0b51adb3&

Suggestion: For outbound call transfers with Telnyx SIP, make sure SIP REFER is enabled on your Telnyx SIP trunk, as Vapi requires this for transfers. In your Telnyx portal, go to your SIP trunk settings and confirm that "SIP REFER" is turned on. If it’s not enabled, outbound transfers will drop after the transfer message. Example:

# Telnyx SIP trunk setting
Enable: SIP REFER (Call Transfer)

If SIP REFER is already enabled and the issue persists, check your SIP logs for REFER packets or review [Vapi's SIP trunk troubleshooting guide](https://docs.vapi.ai/phone-calling/transfer-calls/debug-forwarding-drops). Sources: - [Vapi documentation: call forwarding](https://docs.vapi.ai/call-forwarding) - [Vapi SIP Trunking Telnyx external number call forwarding troubleshooting](https://discord.com/channels/1211482211119796234/1343963959421829172) - [Vapi SIP trunk documentation](https://docs.vapi.ai/advanced/sip/sip-trunk)

Hi prum8439, If you need to set up and configure call forwarding using Vapi's tools, you have a few options depending on your setup preferences and requirements: 1. **Dashboard Configuration**: You can set up call forwarding by creating a Transfer Call Tool in the Vapi dashboard. Navigate to Tools, create a new tool, select "Transfer Call" as the tool type, and then configure your destination numbers and messages. 2. **API Configuration**: Define the tool via API to specify destination numbers and corresponding messages. Include parameters like type, number, and messages in your configuration. 3. **Dynamic Transfers**: For real-time routing decisions based on conversation context, use dynamic call transfers. This involves setting up a webhook to determine the transfer destination during a call. 4. **Warm Transfer Modes**: Implement warm transfers for better context-handling during transfers. Modes like "warm-transfer-with-summary" and experimental options ensure the receiving end has context before picking up the call. 5. **Assistant-Based Transfers**: Configure AI assistants to manage transfers. The dedicated assistant handles interactions with the operator, makes decisions to either complete or cancel transfers, and can handle multiple departments with specific prompts. For detailed steps on implementing these methods, please refer to the [call forwarding documentation](https://docs.vapi.ai/call-forwarding) and [assistant-based warm transfer documentation](https://docs.vapi.ai/calls/assistant-based-warm-transfer).

I’ve already set up the transfer call, and as you can see in the screenshot, it works successfully. But after the call gets the transfer call status, the call just drops.

Hello. We are truly sorry for the inconvenience can you try these steps to troubleshoot calls that drop immediately after transfer?: 1. First, check the

endedReason

in your call logs: - If it shows "assistant-forwarded-call" - Vapi initiated the transfer correctly - Any other value - Check your assistant configuration[1](https://docs.vapi.ai/phone-calling/transfer-calls/debug-forwarding-drops) 2. Common configuration issues that cause drops:

{  "assistant": {    "serverMessages": ["phone-call-control", "status-update"]  // Remove "phone-call-control"  }}

- Remove

"phone-call-control"

from

serverMessages

if present - Set

phoneCallProviderBypassEnabled

to

false

3. If still dropping, verify: - Destination number format includes country code - Test calling the destination directly outside of Vapi - Check if destination has call blocking enabled - Ensure transfer scenario is supported: - Phone to phone: ✅ Supported - Web to phone: ❌ Not supported (will always drop) - PSTN to SIP: ❌ Not supported 4. If the above doesn't help: - Check your telephony provider logs for transfer failures - Analyze call records for forwarding errors - Test with different destination numbers

Hi, I've tried all the troubleshooting steps you provided: ✅ Checked endedReason in call logs - shows "assistant-forwarded-call" (Vapi initiated transfer correctly) ✅ Removed "phone-call-control" from serverMessages ✅ Set phoneCallProviderBypassEnabled: false at call level ✅ Verified destination number format with country code ✅ Tested calling destination directly - works fine ✅ Confirmed transfer scenario is Phone to phone (supported) The issue persists - all outbound transfers still drop immediately after transfer attempt. Through PCAP analysis, I found the root cause: Vapi SBC (44.238.177.138) responds with 503 Service Unavailable to SIP REFER requests from Telnyx. This appears to be a systemic issue with Vapi+Telnyx SIP REFER integration for outbound calls. Dynamic Transfer works fine (uses webhooks), but standard transfer fails consistently. Is this a known limitation of the Vapi+Telnyx integration? Any ETA on SIP REFER support fix?

Could you please provide the call id where this occurred?

d7757822-33e9-497a-b847-c84b1c920ebe

Thank you for providing this information. We’ll be looking into it further to better understand the situation and determine the next steps. Our team will update you as soon as we have more details, and we truly appreciate your patience and cooperation in the meantime

any update ?

We are still investigating this matter. We appreciate your patience with us

any update ?

We’ll share an update as soon as we have more information. Thank you for bearing with us and for your patience while we work to get everything back to normal.