To resolve the "unknown-error" when using your transfer tool, ensure your tool's parameters include a properly configured

destination

property. Here’s a sample configuration you should use:

json
"parameters": {
  "type": "object",
  "properties": {
    "destination": {
      "type": "string",
      "enum": ["+1234567890"], // Replace with your external number in E.164 format
      "description": "The destination to transfer the call to."
    }
  },
  "required": ["destination"]
}

Update your tool with this structure and test again. If the error persists, check your call logs and contact Vapi support with your call ID for further help. [Reference: Vapi documentation – Call Forwarding](https://docs.vapi.ai/call-forwarding) Sources: - [Vapi documentation: call forwarding](https://docs.vapi.ai/call-forwarding) - [Vapi Discord support thread](https://discord.com/channels/1211482211119796234/1372460612499804270)

{ "id": "fd5f90c4-26b2-4e1f-8e53-2aadfc4487b9", "createdAt": "2025-05-21T18:49:08.953Z", "updatedAt": "2025-05-21T18:52:16.134Z", "type": "transferCall", "function": { "name": "Transfer_Do_Operatera", "description": "Ova funkcija se pokrece kada korisnik zeli da se razgovara sa operaterom.", "parameters": { "type": "object", "properties": {}, "required": [] } }, "messages": [ { "type": "request-start", "blocking": true } ], "orgId": "767ee540-d419-4f38-866e-082470b5c4f8", "destinations": [ { "type": "number", "number": "+381652221200", "message": "Molim Vas sacekajte par sekundi, sada cu da preusmerim poziv ka dostupnom operateru.", "description": "Korisnik zahteva da prica sa operaterom onda se preusmerava poziv ka operateru.", "transferPlan": { "mode": "warm-transfer-say-summary", "sipVerb": "refer", "summaryPlan": { "enabled": true, "timeoutSeconds": 5, "useAssistantLlm": true } }, "numberE164CheckEnabled": true } ] } Here is the code

this is also the function id: fd5f90c4-26b2-4e1f-8e53-2aadfc4487b9

I can provide you with more call ids, I've called over 10 times testing this function but each time the call just ends

Hey Katovi, you cannot transfer to someone's phone number or do WARM transfer using webcalls. WARM transfer and transferring to another phone number or SIP number is only limited using phone calls for now, but we want to add it for webcalls as well. I'll request you to raise a feature request for it.

Like what can I use that function for then

The

transferCall

tool is intended for transferring phone calls (not webcalls) to real phone numbers or SIP endpoints. Currently, this feature is only supported for phone-based calls—if you try to use it in a webcall, the transfer will fail. If you need this for webcalls, you can submit a feature request at [roadmap.vapi.ai](https://roadmap.vapi.ai). For now, use the transfer tool for routing live phone calls only, as documented in the [Vapi call forwarding guide](https://docs.vapi.ai/call-forwarding). Sources: - [Vapi documentation: call forwarding](https://docs.vapi.ai/call-forwarding) - [Vapi Discord support thread](https://discord.com/channels/1211482211119796234/1372460612499804270)

Currently, the

transferCall

tool in Vapi is mainly useful for transferring phone calls to other numbers, SIP endpoints, or assistants—but this is only supported for phone calls, not webcalls. For webcalls, you can use the tool to hand off the conversation between assistants within a squad (for example, moving from a sales assistant to a payment assistant), but you cannot transfer to an external phone number or SIP address yet. Example use case for webcalls:

json
{
  "type": "transferCall",
  "destinations": [
    {
      "type": "assistant",
      "assistantName": "PaymentAssistant"
    }
  ]
}

This will silently transfer the webcall to another assistant within your Vapi squad. External transfers for webcalls are not yet supported.[Learn more](https://docs.vapi.ai/squads/silent-transfers) Sources: - [Silent transfers](https://docs.vapi.ai/squads/silent-transfers) - [Call Forwarding](https://docs.vapi.ai/call-forwarding) - [Custom LLM Tool Calling Integration](https://docs.vapi.ai/customization/tool-calling-integration)

The

transferCall

tool is intended for transferring calls to phone numbers, SIP endpoints, or other assistants—but this only works for phone calls, not webcalls. For webcalls, transferring to a real phone number or performing a warm transfer is not currently supported, so the tool cannot be used for that purpose. You can use

transferCall

for internal routing between assistants in a squad or for SIP transfers, but only in supported call types (i.e., phone calls). If you need this feature for webcalls, consider submitting a feature request as suggested by support. Sources: - [Call Forwarding (transferCall tool)](https://docs.vapi.ai/call-forwarding) - [Default Tools: Transfer Call](https://docs.vapi.ai/tools/default-tools#transfer-call) - [Silent Transfers (Squads)](https://docs.vapi.ai/squads/silent-transfers)