The Call Transfer tool lets your AI agent transfer an active telephony call to a phone number or SIP endpoint. You can configure a fixed destination, use a template from call context, or resolve the destination at transfer time by calling your HTTP endpoint. Use this tool when the caller should speak to a human operator, department, escalation queue, support queue, or another phone system.

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Supported providers

Call transfer is available for telephony calls using Twilio, Telnyx, or Asterisk ARI providers.
Web calls do not support call transfer.

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How call transfer works

When the LLM decides a transfer is needed, it calls the Call Transfer tool. EESI resolves the destination, validates the transfer setup, and then starts the provider transfer. For a successful transfer:
  1. The agent calls the Call Transfer tool.
  2. EESI resolves the destination from either static configuration or a dynamic HTTP resolver.
  3. EESI optionally plays a pre-transfer message.
  4. EESI starts dialing the destination through the telephony provider.
  5. The caller hears hold music while the destination leg is connecting.
  6. When the destination answers, EESI connects the caller and ends the AI agent’s involvement.
The transfer is a blind transfer. EESI does not currently pass conversation context to the receiving human as part of the transfer.

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Destination source

You must choose one destination source for each Call Transfer tool.

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Static / Template

Use this when the transfer destination is already known before the transfer happens. Static destinations can be:
  • A fixed E.164 phone number, such as +1234567890
  • A SIP endpoint, such as PJSIP/sales-queue
  • A template value from context, such as {{initial_context.transfer_destination}}
This is the simplest option for fixed support lines, fixed departments, or cases where the destination is provided before the call starts.

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Dynamic HTTP Resolver

Use this when the destination must be decided during the conversation. EESI sends a POST request to your resolver endpoint before starting the transfer. Your endpoint returns the destination and may return a custom message to play before dialing. Common use cases:
  • Route by region, timezone, or language
  • Look up the right account team in your customer system
  • Route by customer tier or account status
  • Route by issue type, language, or department
  • Use business logic from your backend instead of hardcoding numbers in EESI

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Dynamic resolver request

The resolver request body is a flat JSON object. EESI builds it from:
  • LLM parameters: values the agent extracts from the conversation when it calls the transfer tool
  • Preset parameters: values EESI injects from fixed values or templates such as {{initial_context.account_id}} or {{gathered_context.billing_issue_type}}
If both define the same key, preset parameters take precedence. Example request body:
EESI does not send the full conversation transcript to the resolver. Add the specific values you need as LLM parameters or preset parameters.

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Dynamic resolver response

Your resolver must return a JSON object with transfer_context.destination. transfer_context.custom_message is optional.
Follow this response shape exactly. EESI reads specific keys from the resolver response. For dynamic transfer resolution, an invalid response does not fall back silently: the transfer fails before dialing.
Field behavior:
  • destination: Required. E.164 phone number or SIP endpoint.
  • custom_message: Optional. If returned, this message is played before the provider transfer starts.
If the resolver fails, times out, returns invalid JSON, or omits transfer_context.destination, the transfer fails gracefully and the agent can continue handling the caller.

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Dynamic resolver configuration

When using a dynamic resolver, configure:
  • Resolver URL: HTTPS or HTTP endpoint EESI calls with POST
  • Resolver Timeout: How long EESI waits for the resolver response
  • Resolver Wait Message: Optional message spoken while EESI waits, such as “One moment while I find the right team.”
  • LLM Parameters: Values the agent should extract from the conversation, such as billing_issue_type or requested_department
  • Preset Parameters: Values injected by EESI from context or fixed values
  • Custom Headers: Static headers sent to your resolver endpoint
  • Credential: Optional saved credential used to authenticate the resolver request
The dynamic resolver uses POST only. If you need custom backend behavior, implement it behind your resolver endpoint and return the expected transfer_context shape.

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Pre-transfer messages

Call Transfer supports a common pre-transfer message setting for both static and dynamic destinations. For static transfers:
  1. EESI resolves the static or template destination.
  2. EESI plays the configured pre-transfer message, if any.
  3. EESI starts the provider transfer.
For dynamic transfers:
  1. EESI may play the resolver wait message while calling your resolver.
  2. If the resolver returns custom_message, EESI plays that message.
  3. If the resolver does not return custom_message, EESI uses the configured pre-transfer message.
  4. EESI starts the provider transfer.
Resolver custom_message overrides the configured pre-transfer message for that transfer attempt.

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Transfer timeout

Transfer timeout is the maximum time EESI waits for the destination to answer after the provider transfer starts. This timeout applies to both static and dynamic transfers. The resolver timeout is separate. It controls only how long EESI waits for your HTTP resolver before dialing starts.

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Destination formats

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Twilio and Telnyx

Use E.164 phone numbers:
The number must be reachable from your telephony provider.

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Asterisk ARI

Use SIP endpoints:
Asterisk ARI transfers only work with SIP endpoints configured on your Asterisk server. External phone numbers require PSTN trunk configuration in your Asterisk setup.

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Example: route enterprise billing calls

Use this setup when the agent identifies a billing issue and your backend decides whether to route the caller to standard billing, enterprise billing, or collections. Configure LLM parameters:
Configure preset parameters:
Your resolver receives:
Your resolver returns:

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Best practices

  • Keep resolver latency low. Aim for 2-3 seconds.
  • Use a resolver wait message if the resolver may take noticeable time.
  • Send only the parameters your resolver needs.
  • Prefer preset parameters for values already available in initial_context or gathered_context.
  • Make LLM parameter descriptions explicit so the agent knows what to extract.
  • Test both successful transfer and resolver failure paths before going live.
  • Keep destination validation and routing logic in your backend when routing depends on account data or business rules.

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Troubleshooting

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The transfer tool is not called

Check that the tool is attached to the correct node and that the node prompt clearly tells the agent when to transfer.

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Static transfer fails with no destination

Static mode requires a configured destination. Use a fixed destination or a template that resolves to a non-empty value.

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Dynamic transfer fails before dialing

Check the resolver response. It must include:
Also verify the resolver URL, authentication headers, credential, and timeout.

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Resolver receives missing parameters

Confirm the parameter is configured as either:
  • an LLM parameter, if the agent should extract it from the conversation
  • a preset parameter, if EESI should inject it from context
For known context values, prefer preset parameters like {{initial_context.account_id}} or {{gathered_context.billing_issue_type}}.

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Destination does not connect

Verify that the phone number or SIP endpoint is valid and reachable from the configured telephony provider.