This feature only works during phone calls instead of web calls. It’s available for Retell numbers and imported numbers.
It is common in call operation to transfer the call to another human agent or another AI agent.
1

Add Transfer Call Tool

Click ”+ Add” in the tools section and select “Transfer Call” from the dropdown menu.
2

Configure Basic Settings

  • Define specific conditions under which the call should be transferred. For example, if the user is angry, transfer the call to a human agent.
  • For the transfer destination, you can choose between a static number (e.164 or SIP URI), or the value of a dynamic variable.
3

Update the Prompt

Enhance the agent’s understanding by incorporating the transfer call conditions into the prompt. Include specific instructions such as:“If the user is angry or frustrated, use the transfer_call tool to transfer the conversation to a human agent.”
4

Configure Transfer Type

Choose between cold transfer or warm transfer:
  • Cold transfer: The call is transferred to a destination number and that’s it.
  • Warm transfer: After the call is transferred to the destination number, the AI agent attempts to first detect if the other side is a human, then speaks private and public messages (more details in Step 5).
5

Configure Caller ID for Cold Transfer (Optional)

For cold transfers, you can configure how the caller ID appears to the next agent:
  1. Retell Agent’s number: The transfer destination will see the Retell agent’s number
  2. Transferree’s Number: The transfer destination will see the number that the user is calling from. Please note that you have to configure following in your telephony provider in order to make it work, because this option internally is simply transferring via SIP REFER, and some telephony provider have the ability to set caller id for SIP REFER.
    • enable SIP REFER
    • enable transfer to PSTN
    • set caller id as transferee
6

Configure Settings for Warm Transfer

For warm transfers, you can configure the following settings:
  • On-hold music: The audio played to the caller while they are on hold. The default is a standard ringtone.
  • Agent detection timeout: The maximum amount of time the AI agent will wait to determine whether the transfer target is a human. The caller is connected only if human detection succeeds within this timeframe. Otherwise, the transfer is marked as failed. The default timeout is 30 seconds.
  • Whisper message (optional): A message spoken privately to the transfer target before connecting them to the original caller.
  • Three-way message (optional): A message spoken to both the transfer target and the original caller once the connection is established.
7

Add Custom SIP Headers (Optional)

Add custom SIP headers for outbound calls. These headers are forwarded to your SIP provider on the SIP INVITE and can be used for custom routing, tagging, or metadata.All header names must start with X-