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The [TRANSFER:] tag forwards the active call to a different phone number. Nixflex supports both cold and warm transfers.

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Cold transfer (default)

The agent says a short handoff line, then connects the caller directly to the destination. Fast, simple. 
[TRANSFER:+442087601234]
Flow:
  1. Claude says: “Hold on, I’ll put you through to our manager now.”
  2. Engine closes its audio stream
  3. Twilio dials the destination
  4. When picked up, caller and destination are connected
  5. Nixflex steps out
The destination doesn’t hear any context. They pick up a normal call from the caller’s number.

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Warm transfer

The agent dials the destination first, briefs them in a private voice (the caller can’t hear this), then merges everyone into the conversation. 
[TRANSFER:+442087601234:warm]
Flow:
  1. Claude says: “Let me get the manager on the line for you, one moment.”
  2. Engine puts the caller on hold (music or silence)
  3. Engine dials the destination
  4. When the destination picks up, Claude speaks a private briefing — “Hi, I have Sarah on the line. She’s asking about a refund for an order placed last Tuesday. Putting her through now.”
  5. After the briefing, the caller is merged in and the three-way conversation begins
  6. Engine steps out once everyone is connected
Warm transfers help the human pick up context instantly. Useful for complex handoffs.

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Configuring the destination

Two options for where transfers go: Inline in the prompt: 
ACTIONS
- For refund requests, transfer to the manager: [TRANSFER:+442087601234]
- For technical questions, transfer to support: [TRANSFER:+442087601235]
- For clinical questions, warm transfer to the dentist: [TRANSFER:+442087601236:warm]
Default transfer number on the agent: Set transfer_number when creating the agent. Then Claude can use the bare tag: 
[TRANSFER:default]
The engine substitutes the agent’s transfer_number field.

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When to use which

Cold transfer

Use when: the destination already knows what to expect (a generic support line, a manager who handles all overflow).Faster but less context for the receiver.

Warm transfer

Use when: the receiver needs context (a specialist, a senior staff member, complex situations).Slower but higher-quality handoff.

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Failure handling

If the destination doesn’t pick up (busy, voicemail, no answer):
  • Cold transfer: the caller is dropped to voicemail or hangs up
  • Warm transfer: the caller stays on the line with Claude, who can say “I couldn’t reach them, can I take a message?”
You don’t have to script this. Claude handles it naturally.

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Telling Claude when to transfer

Be specific in the prompt about transfer triggers: 
RULES
- Never give medical advice. If the caller has a clinical question, transfer immediately: [TRANSFER:+442087601236:warm]
- If the caller asks for a manager, transfer: [TRANSFER:+442087601234]
- If you don't know the answer, transfer rather than guess: [TRANSFER:+442087601234]
Claude reads these rules every turn and applies them automatically.