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Call transfers allow your AI agents to seamlessly hand off conversations to human operators when needed. This is essential for scenarios where the AI agent cannot resolve a customer’s issue, needs to escalate to a specialist, or when the caller specifically requests to speak with a human.

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Understanding Call Transfers

Call transfers work by providing your AI agent with a custom tool that enables it to transfer active calls to human operators. The agent uses this tool based on instructions in your system prompt and the tool’s description, determining when and how to initiate transfers.
Agent-Initiated TransfersThe AI agent decides when to transfer calls based on your instructions. You control this behavior through your system prompt and tool configuration.

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Transfer Variants

There are two main types of call transfers you can implement: Cold Transfer → The call is immediately transferred to the human operator without any preparation or context sharing. Also known as a blind transfer or unattended transfer. Warm Transfer → The agent provides context about the conversation to the human operator before the caller is connected. Also known as an attended transfer or whisper transfer. Additionally, there are two methods by which a transfer may be implemented: Refer → The SIP REFER verb is used to connect the caller with the human operator, removing the agent and related SIP stack altogether. This is almost always preferable as long as all the relevant SIP providers support it. Bridge → The human operator is dialed using a second SIP INVITE. While the agent leaves the call, the SIP stack remains and bridges audio between the caller and the human operator. This is less efficient and will incur additional costs, but may be necessary if your SIP provider does not support REFER transfers.
Bridge Transfers Cost MoreUsing bridge transfers will result in SIP charges beyond the Ultravox call duration as our SIP infrastructure remains active to bridge audio between the caller and human operator.

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SIP Call Transfers

For SIP calls, transfers can be achieved using built-in tools. Warm transfers are coming soon. For cold transfers, use the coldTransfer tool. The built-in tools use refer transfers by default. If you need to use bridge transfers instead, you can override the sipVerb parameter from REFER to INVITE when adding the tool to your agent (or call). You may also wish to set from, username, and/or password in order to authenticate the subsequent INVITE. Should the transfer fail (e.g., the human operator declines the call or otherwise fails to answer), the caller will be returned to the AI agent to continue the conversation. Transfers may be retried by the agent as needed.

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Call Transfers with Other Telephony Providers

Built-in coldTransfer tool with Twilio
The built-in coldTransfer tool also works with Twilio if you’ve added your credentials, but be aware that invoking the tool will immediately end the Ultravox call regardless of whether the transfer succeeds.
To implement call transfers:
1

Create a Transfer Tool

Build a custom tool that your AI agent can call to initiate transfers. This tool should handle the telephony provider’s transfer APIs.
2

Configure Agent Instructions

Update your system prompt to instruct the agent when and how to use the transfer tool. Include guidelines for all transfer scenarios and when transfers should occur (e.g., “Transfer when you cannot answer billing questions” or “Transfer if the customer asks for a manager”).Instruct your agent to politely explain the transfer to the customer before initiating it (e.g., “I’m going to connect you with a specialist who can better help with your billing question”).
3

Handle Transfer Logic

Implement the backend logic to manage the actual call transfer using your telephony provider’s APIs.
Example Call Transfer Tool Definition
{
  "toolName": "transferCall",
  "description": "Transfer the current call to a human agent when you cannot resolve the customer's issue or when they specifically request to speak with a human.",
  "parameters": {
    "destinationNumber": {
      "type": "string",
      "description": "The phone number to transfer the call to"
    },
    "transferReason": {
      "type": "string",
      "description": "Brief explanation of why the call is being transferred"
    }
  }
}

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Twilio

Twilio supports both blind and attended transfers through different APIs. Blind transfers use the simple <Dial> verb to immediately connect the caller to a new destination, while attended transfers utilize Twilio’s Conference API to create a three-way call before the agent disconnects. Blind Transfer: Uses Twilio’s calls.update() method with TwiML containing a <Dial> verb to immediately redirect the call. Attended Transfer: Creates a conference call, places the original caller on hold, calls the human agent with a whisper message, and allows the agent to join the conference after hearing the context. The attended transfer process involves:
  1. Putting the caller on hold with music
  2. Creating a conference call
  3. Calling the human agent with the transfer reason
  4. Requiring the agent to press a key to join
  5. Connecting all parties to the conference
  6. Allowing the AI agent to disconnect
Complete Example AvailableWe have a full working example of Twilio call transfers (including both blind and attended transfers) available in the ultravox-examples repo.

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Conclusion

Call transfers are essential for creating robust AI-powered voice applications that can seamlessly escalate to human operators when needed. By implementing both blind and attended transfer capabilities, you can ensure customers receive the appropriate level of service while maintaining a smooth experience throughout the conversation.