Add the Transfer Call tool to a Retell single or multi-prompt agent to route phone calls to a human or another number — supports cold, warm, and agentic warm transfers.
Add the Transfer Call tool so your agent can hand a live phone call off to a human, a department, or another number when the conversation calls for it.For example, a support agent can stay on the line to answer routine questions and transfer to a human the moment a caller asks for a refund or sounds frustrated.
Transfer Call works on phone calls only, not web calls. It’s available for Retell numbers and imported numbers.
To hand the conversation to another AI agent instead of a phone number, use Agent Transfer — it’s lower-latency and passes the full conversation history to the next agent.Building a conversation flow agent instead? Use the call transfer node, which supports the same transfer types.
1
Add the Transfer Call tool
In the tools section, click + Add and select Transfer Call from the dropdown.
2
Set the transfer target
Set the transfer number to either:
a number in E.164 format, or a SIP URI in the form sip:username@domain (for example sip:user@retellai.com), or
(Optional) If your transfer destination isn’t in E.164 format, choose raw format to keep the input as is. This applies only when you use custom telephony, not Retell Telephony.
Enter the transfer number in E.164 format, or use a dynamic variable.
Set the transfer number extension if needed. The extension can contain 0-9, *, and # (for example 123#).
3
Update the prompt
Tell the agent when to transfer by adding the condition to its prompt. For example:
If the user is angry or frustrated, use the transfer_call tool to transfer the conversation to a human agent.
4
Choose the transfer type
Choose how the call is handed off. Think of it like passing a call to a colleague:
Cold transfer: The call goes straight to the destination and your agent drops off right away — like transferring a call and hanging up.
Warm transfer: Your agent stays on the line after dialing the destination. It can check whether a real person answered, play a private message only the destination hears, or do a quick three-way introduction before connecting your caller (details below).
Agentic warm transfer: A second AI agent — the transfer agent — picks up the handoff, has a short back-and-forth with the destination, then decides whether to connect (bridge) your caller or cancel. Use this when you want an AI teammate to screen or brief the destination first (details below).
5
Set the dial timeout (optional)
Set how long the destination rings before the attempt counts as unanswered. This applies to cold, warm, and agentic warm transfers.The value you set here applies only to this transfer. If you leave it unset, the transfer uses your agent-level ring duration.
Set how long the destination rings before the transfer counts as unanswered.
6
Configure caller ID (optional)
Choose which caller ID the transfer destination sees:
Retell agent’s number: The destination sees the Retell agent’s number.
User’s number: The destination sees the caller’s number. Your telephony provider must support caller ID override for this to work.
Warm transfer uses SIP DIAL, setting the from and P-Asserted-Identity headers to the user’s number.
Cold transfer uses SIP REFER; supporting caller ID override is up to the telephony provider.
Retell Twilio numbers support showing the user’s number on both warm and cold transfer. Retell Telnyx numbers support it only via cold transfer (SIP REFER).
If caller ID override isn’t supported, the transfer fails.
Choose whether the destination sees the Retell agent's number or the caller's number.
7
Configure cold transfer settings
For cold transfers, choose the SIP method used to hand off the call. SIP (Session Initiation Protocol) is the signaling protocol that sets up and routes VoIP calls.
SIP INVITE: The default. Establishes or updates the active call path, then bridges the transfer. You choose which caller ID the destination sees.
SIP REFER: Asks the endpoint to start a separate call to the destination for the handoff. Use this only if your telephony provider supports SIP REFER; caller ID behavior then depends on the provider.
The User’s number caller ID option applies only when the cold transfer mode is SIP INVITE.
Choose SIP INVITE or SIP REFER for the cold transfer handoff.
8
Configure warm transfer settings
For warm transfers, you can configure:
On-hold music: What the caller hears while on hold. Defaults to a standard ringtone.
Navigate IVR: A prompt to help the agent navigate an IVR if the destination is an IVR system.
Enable human detection: Check whether a human is present after the destination answers. The caller is connected only once a human is detected.
Auto-greet: The agent says “Hello” as soon as the destination picks up, prompting a response so a human is easier to detect.
Agent detection timeout: How long the agent waits to decide whether the destination is human. The caller is connected only if detection succeeds within this window; otherwise the transfer is marked failed. Defaults to 30 seconds.
Whisper message (optional): A message spoken privately to the destination before the caller is connected.
Three-way message (optional): A message spoken to both the destination and the caller once connected.
Warm transfer settings, including human detection and whisper message.
9
Configure agentic warm transfer settings
If you chose agentic warm transfer, a transfer agent handles the handoff. You can configure:
On-hold music: What the caller hears while the transfer agent is working. Defaults to a standard ringtone.
Two-way conversation agent: The transfer agent that talks to the destination and decides whether to connect the caller. See the next step to choose or create one.
Wait time for agent answer: How long to give the transfer agent to reach a decision (connect or cancel).
Action on timeout: What happens when that wait time runs out — Cancel transfer (treat it as failed) or Bridge the transfer (connect the caller anyway).
Three-way ring tone (optional): A short sound played the moment the call connects.
Three-way message (optional): A short handoff message shared with both sides once connected. Write it as a Prompt (instructions the agent turns into a sentence) or a Static Sentence (the exact words to say).
Agentic warm transfer settings, including the two-way conversation agent and timeout behavior.
10
Choose or create a transfer agent
A transfer agent is a separate AI agent whose only job is to receive agentic warm transfers — an AI teammate that answers the handoff, speaks with the destination, and decides whether to connect your caller. Transfer agents are kept separate from your normal agents, so they don’t clutter your main agent list.1. Open the picker. Click the Two-way conversation agent selector (it reads “Transfer screening agent” until one is chosen) to open the Select transfer agent window.2. Pick an existing one. Use the search box to find a transfer agent, click it to see a preview on the right, then click Confirm selection.
Search for a transfer agent, preview it, then confirm.
3. Or create a new one. Click Create new, choose the type — Single Prompt (one set of instructions) or Conversation Flow (a step-by-step visual flow) — then click Create. The new transfer agent opens so you can set it up right away.
Choose a Single Prompt or Conversation Flow transfer agent, then create it.
4. Manage a transfer agent. Hover a selected agent and open its ⋯ menu to Rename it, select a specific Version (otherwise it always uses the latest), or Delete it. Deleting removes all versions and removes the agent from any transfer that was using it.
The transfer agent card menu with Rename, Versions, and Delete.
Once selected, you’ll see the transfer agent’s voice, with an arrow to open and edit that agent in a new tab whenever you need to.
When you open a transfer agent to edit it, it opens in its own tab and a flow selector appears at the top of the builder (showing Main flow). Use it to switch back to your main agent at any time.
11
Add custom SIP headers (optional)
Add custom SIP headers for outbound calls. Retell forwards these headers to your SIP provider on the SIP INVITE, so you can use them for custom routing, tagging, or metadata. See SIP headers for the full reference.
Custom SIP headers are preserved only when transferring directly to a SIP endpoint. They may be stripped when transferring to a PSTN number.
All header names must start with X-, or be User-To-User (case insensitive).
Add custom SIP headers forwarded on the SIP INVITE.
What's the difference between Transfer Call and Agent Transfer?
Transfer Call places a real phone call to another number (a human, a department, or an external line). Agent Transfer hands the conversation to another AI agent within the same call — no new phone call, lower latency, and the destination agent inherits the full conversation history. Use Transfer Call to reach a person or outside number; use Agent Transfer to switch between AI agents.
When should I use warm transfer instead of cold transfer?
Use cold transfer when you just need to route the caller and drop off. Use warm transfer when the agent should confirm a human answered, brief the destination privately, or introduce the caller before connecting. Use agentic warm transfer when you want an AI transfer agent to screen or brief the destination first.
The transfer destination sees the wrong caller ID. What's wrong?
Showing the caller’s number requires caller ID override support from your telephony provider. Retell Twilio numbers support it on both warm and cold transfer; Retell Telnyx numbers support it only via cold transfer (SIP REFER). If override isn’t supported, the transfer fails — switch the caller ID setting to Retell agent’s number or use a supported number.
