VAPI•8mo ago

TransferCall

I’ve successfully SIP Trunked VAPI assistant to Telnyx. The issue I'm experiencing is when testing, call isn't forwarding to external number.

As shown in SS I've created 'TransferCall' tool while inputting the number under predefined functions. Not to mention included number in the prompt. Am I missing something?

Vapi Support BotAPP•5/15/25, 6:30 AM

It looks like your TransferCall tool's "Parameters" section is missing the required "destination" property, which is needed for the call to forward. Try updating your configuration as follows:

"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"]
}

"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"]
}

Add this to your function's parameters section, and the call transfer should work as expected.

Source:

Vapi documentation: call forwarding

⤵️ Mention @Vapi Support Bot for follow-ups 🐥

Vapi Support BotAPP•5/15/25, 6:30 AM

The most common cause for this issue is that Telnyx may not support SIP REFER, which Vapi uses for call transfers. Please check your Telnyx SIP trunk settings to ensure SIP REFER is enabled. If Telnyx support has advised you to disable SIP REFER, note that Vapi currently does not support changing the SIP verb for transfers, so the transfer will fail in that case.
Reference: "Vapi uses SIP REFER for call transfers. If Telnyx or your SIP trunk does not support SIP REFER or requires a different SIP verb, the transfer may fail."

Source:

supportTransfering calls issues with Telnyx

specific-silver•5/15/25, 8:47 AM

DID you enable SIP REFER in Telnyx ?

Sspecific-silver DID you enable SIP REFER in Telnyx ?

wispy-yellowOP•5/15/25, 10:17 AM

Contacted Telnyx support to enable this

Lltd-scarlet It looks like your TransferCall tool's "Parameters" section is missing the requi...

wispy-yellowOP•5/15/25, 10:19 AM

@Nir S (CEO/Founder @Cloudonix) is this format needed under parameters?

specific-silver•5/15/25, 10:24 AM

Which format? The URI format?

Sspecific-silver Which format? The URI format?

wispy-yellowOP•5/15/25, 11:51 AM

JSON format

specific-silver•5/15/25, 11:54 AM

What do you mean? The JSON for the tool?

VapiAPP•5/15/25, 1:18 PM

Hey,

Could you please schedule a brief call at your convenience so we can discuss this matter in detail?

Kindly use the following link to book a suitable time: https://calendly.com/shubham-vapi/chat-30m.

Sspecific-silver What do you mean? The JSON for the tool?

wispy-yellowOP•5/15/25, 2:23 PM

Yes, under parameters

"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"] }

TransferCall

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