Hello support, We have raised this previously and it has not been resolved. I will therefore recap the issue below as we have tried many different things including some tips from you but the issue is still persistent. The flow of the call: My mobile phone to a Vonage number - that triggers the assistant. Assistant triggers callforward. The transfer was to another Vonage number/assistant/mobile phone. The agent is using custom tranferCall tool. The conversation goes fine and the assistant tried to forward the call with the error: call.in-progress.error-transfer-failed. We are using Vonage for our assigned numbers (both for the number being called in and for some of the forwarding numbers) Latest call IDs: 4c158b40-d0fa-4519-a969-1f94dd3b1c44; 1bc0ef0c-43d6-472b-a1dd-19fff3792259 We created custom tool for transferCall. During our testing we have tried using external Vonage numbers, assistant forwarding, using real mobile numbers for the destinations. In the prompt we have followed the best practises suggestions and have tried many reinforcing methods and alternatives to ensure its not prompting issue We have tried different Open AI models to rule out that to be an issue. We have added the enums into the tool (something that had to be done via API) as that was suggested by your support but that did not make any difference either. I have also recreated the whole setup in a separate account using the free US Vapi number but still seeing the same issue. We have tried to raise this before here, but we are getting nowhere, so I thought its worth doing the full context to the issue in one place, hence the above. https://discord.com/channels/1211482211119796234/1402646499292352542 If you think this might be an issue with Vonage, can you please let us know what we should be investigating? As far as we can tell, the setup is correct and we are unsure of what exactly we should be checking.

Thank you for providing the call id. We will review the logs and get back to you with our findings

Any update on this please?

Hey there, Sorry for the delayed response. We checked the logs and it appears that the destination ending in 5619 was attempted with

warm-transfer-experimental

where warm transfers are Twilio exclusive transfers. Please switch that phone number destination transfer type to

blind-transfer

I've already tried various transfer types, but they all get disconnected. Is there something on the Vonage side we can check, settings-wise? There are no call logs in Vonage.

Can you check which SipVerbs they accept for transfers? We typically use

REFER

, but we also support

DIAL

and

BYE

for other transfer methods.

INVITE, ACK, BYE, CANCEL, REGISTER, OPTIONS → Supported (core SIP). REFER → Generally not supported for mid-call transfers via Vonage trunks (instead, you’d handle transfers at the PBX level and re-INVITE through the trunk). DIAL → Not a SIP method. This is a Twilio-style verb. On Vonage trunks, you would achieve the same by issuing an INVITE to the trunk.

With that being said, please try configuring the transferCall tool to use

BYE

as the sipVerb instead of

REFER

. We can also inspect a more recent call id for the PCAP file if there are any errors in that.

I am having the same issue with transfers using twilio. I have been discussing this issue with Twilio and they said: "Can you please ask the VAPI team how they handle SIP REFERs? The current blocker impacting your SIP REFERs is related to the VAPI system, there is a via on the forwarded calls, and those calls are using your mobile number in testing. Can you ask VAPI to help you pass only Twilio's caller-ID when SIP REFER is invoked?" They wanted to schedule a zoom between VAPI, Twilio, and myself but I understand that isn't possible. I am trying to change the sipVerb to BYE per your suggestion but I am struggling to do so. I am reviewing the documentation for warm transfers but I am struggling with creating a custom tool that uses that verbage. Is there any premade templates for this that I can copy from

Thank you for sharing these details along with Twilio’s feedback. Our team is actively looking into this, and we’ll continue investigating. To help us move forward, could you please provide your Org ID so we can review this directly on your account? Once we have that, we’ll be able to dig deeper and guide you on the next steps. We appreciate your patience and will update you as soon as we have more information.

Org ID 60fd92b0-5773-4093-ae44-8f00eaad59f5

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.