Short answer: transfer_number is the required argument the agent passes to the transfer_to_number system tool when performing a human call transfer. It must exactly match one of the phone numbers (or SIP URIs, when using SIP REFER) you configured in that agent’s “Transfer to Human” tool rules.
How to use it
Enable the tool: In your agent, add the “Transfer to Human” system tool (transfer_to_number). (elevenlabs.io)
Configure rules: Add one or more destinations and conditions:
Destination format: phone numbers must be E.164 (e.g., +12125551234). SIP URIs must be sip:... and require SIP REFER transfers. (elevenlabs.io)
Choose transfer type: Conference (default) or SIP REFER (needed for SIP URI transfers). (elevenlabs.io)
LLM call: When conditions are met, the model calls transfer_to_number with:
transfer_number: the configured destination (must match your rule),
client_message: what the caller hears during transfer,
agent_message: context for the human recipient,
reason (optional). Example arguments are shown in the docs. (elevenlabs.io)
Notes and constraints
Only available for phone calls (not the chat widget). (elevenlabs.io)
Works with Twilio numbers and SIP trunks; ensure destinations are correctly formatted and permitted by your setup. (elevenlabs.io)
If you configure via API/SDK, add the transfer_to_number system tool and its transfers; at runtime the LLM must pass transfer_number equal to one of those configured values. (elevenlabs.io)
Reference pages
Transfer to human (transfer_to_number): parameters, examples, UI and API setup. (elevenlabs.io)
System tools overview: parameter list confirms transfer_number is required and must match configured numbers. (elevenlabs.io)
Custom LLM integration: shows the function-call format and required arguments
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