BuildActionsCall Transfers

Phone Number Transfers (TEL)

Transfer calls to standard phone numbers in E.164 format.

Phone number transfers (TEL) enable your AI agent to transfer calls directly to a standard phone number. This is the most common transfer type, ideal for routing calls to mobile phones, landlines, or call center queues.

Looking for other transfer types? See the Call Transfers overview for SIP and dynamic transfer options.

Key Settings for Call Transfer

1. Phone Number to Call

  • Specify the phone number that the assistant will transfer the call to. This can be a direct line or an extension within your organization. There is a validation button to confirm that the number is valid.

2. When Should the Transfer Take Place?

  • This section defines the condition for the transfer. For example, you can set it to transfer when the user requests to speak to a manager or human agent.

3. Background Music

  • Optionally enable background music during the transfer process to ensure a smooth and professional experience for the user while waiting for the transfer to be completed.

Transfer Modes

ModeDescriptionBest For
Blind (Cold)Transfer immediately without speaking to the agentSimple routing, queues, IVR systems
Warm (Message)Play a private whisper message to the agentProviding specific context or instructions
Warm (Summary)AI summarizes the conversation for the agentComplex calls requiring full context

Whisper messages are heard only by the receiving agent, not the caller. Use this to brief the agent on the caller’s issue, mood, or any relevant account details before they accept the call.

Advanced Settings

In the Advanced Settings section, you can fine-tune the behavior of the call transfer action:

SettingDescription
ExtensionDTMF digits to dial after connecting (e.g., 123 or 123#). Supports 0-9, *, and #.
TimeoutHow long (in seconds) to wait for the agent to answer. Default: 30 seconds.
Pre-message pauseDelay before the whisper message plays, giving the agent time to settle.
Post-message pauseDelay after the whisper message, before connecting the caller.

Hold Music

Enable background music to keep callers engaged while waiting for the transfer to complete. This provides a professional experience and reassures callers that they haven’t been disconnected.

To enable hold music:

  1. Toggle Background Music in the transfer action settings
  2. The default hold music will play while the recipient’s phone rings

Hold music stops automatically when the receiving party answers and the call is connected.

Record the Call

By default, the recording stops once the call is transferred. If you want to continue recording the call, set the stop_recording_after_transfer parameter to false.

curl
$curl --location --request PUT 'https://api.synthflow.ai/v2/actions/<action_id>' \ 
>--header 'Authorization: Bearer <api_key> \ 
>--header 'Content-Type: application/json' \ 
>--data '{ "LIVE_TRANSFER": { "stop_recording_after_transfer": false 
>} 
>}'

Transfer Failure Handling

What happens when a transfer fails depends on the transfer mode:

Failure ScenarioBlind TransferWarm Transfer
No answer (timeout)Call endsAgent can retry or continue
Busy signalCall endsAgent can retry or continue
Invalid numberCall endsAgent notified, can try alternate
Network errorCall endsAgent can retry or continue

Blind transfers cannot recover from failures. If reliability is critical, use warm transfers to allow graceful fallback behavior.

For warm transfers, you can configure fallback behavior:

  • Have the agent apologize and offer to take a message
  • Try an alternate transfer destination
  • Schedule a callback for later

FAQs

How does Caller ID work?

Transfer ModeCaller ID ShownSIP Method Used
Blind (Cold)Original caller’s numberSIP REFER
Warm (Message/Summary)Synthflow agent’s numberSIP DIAL

Why the difference? Blind transfers use SIP REFER, which hands off the call entirely to your telephony provider, preserving the original caller ID. Warm transfers create a new outbound call leg (SIP DIAL) from Synthflow, so the agent’s number is shown.

If you’re transferring to a PBX or using a custom SIP trunk, ensure your system accepts SIP REFER requests for blind transfers to work correctly.

What happens if a blind transfer fails?

The call between the AI agent and the customer ends immediately. There is no opportunity to retry or recover. If you need failure recovery, use a warm transfer instead.

Can I pass caller information to the receiving agent?

Yes, with warm transfers:

  • Whisper message: Speak a custom message only the agent hears
  • Context summary: AI generates a summary of the conversation
  • Custom SIP headers: Pass structured data to your PBX (for SIP transfers)