Event Types

Complete reference for all events sent by the Flow service.

Overview

Events are JSON objects sent from the Flow service to your application. All events include a type field and session information.

Base Event Structure

All events include session information:

{
  "session": {
    "id": "550e8400-e29b-41d4-a716-446655440000",
    "account_id": "account-123",
    "phone_number": "1234567890",
    "direction": "inbound",
    "from_phone_number": "9876543210",
    "to_phone_number": "1234567890"
  }
}

The direction field indicates whether the call was initiated by the caller ("inbound") or by the Flow via the outbound call API ("outbound"). Use it in your session_start handler to tailor the greeting accordingly.

Event Types

Event Type	Transport	Description	When Triggered
session_start	HTTP + WebSocket	Call session begins	When a new call is initiated
user_speech_started	WebSocket only	Speech onset detected	When VAD detects the user starting to speak (before full transcript)
user_speak	HTTP + WebSocket	User speech detected	After speech-to-text completes (includes barged_in flag if user interrupted)
dtmf_received	HTTP + WebSocket	DTMF digit pressed	When the user presses a key on their phone keypad
assistant_speak	HTTP + WebSocket	Assistant finished speaking	After TTS playback completes
assistant_speech_ended	HTTP + WebSocket	Assistant finished speaking	After speech playback ends
user_input_timeout	HTTP + WebSocket	User input timeout reached	When no speech detected after timeout
session_end	HTTP + WebSocket	Call session ends	When the call terminates
sms_failed	HTTP + WebSocket	SMS delivery failed	After a send_sms action fails — includes reason so the agent can react

Quick Reference

• Session Start - Call begins
• User Speech Started - Speech onset detected (WebSocket only)
• User Speak - User speaks (includes barge-in detection)
• DTMF Received - User pressed a phone key
• Assistant Speak - Assistant speaks
• Assistant Speech Ended - Assistant finished speaking
• User Input Timeout - Timeout reached waiting for user
• Session End - Call ends
• SMS Failed — emitted when a send_sms action fails; see below.

SMS Failed

Emitted to your webhook / WebSocket when a send_sms action fails. The call continues normally — handle this event to react conversationally (e.g. apologize, retry with a corrected number).

{
  "type": "sms_failed",
  "session": { "id": "550e8400-...", "account_id": "...", "phone_number": "...",
                "from_phone_number": "...", "to_phone_number": "..." },
  "recipient": "4915112345678",
  "reason": "sender_not_allowed",
  "message": "SMSC returned faultCode 403"
}

Field	Type	Description
type	string	Always "sms_failed"
session	object	Standard session info
recipient	string	Phone number that failed (the phone_number from your send_sms action)
reason	string	One of: sender_not_allowed, insufficient_balance, no_sms_extension, smsc_unavailable, unknown
message	string	Optional human-readable detail (safe to log, may contain technical error text)

See Send SMS Action for details on each failure reason.

Event Flow

Handling Events

HTTP Webhook

@app.route('/webhook', methods=['POST'])
def webhook():
    event = request.json
    event_type = event['type']

    if event_type == 'session_start':
        # Handle session start
        pass
    elif event_type == 'user_speak':
        # Handle user speech
        pass
    # ... handle other events

WebSocket

ws.on('message', (data) => {
  const event = JSON.parse(data.toString());

  switch (event.type) {
    case 'session_start':
      // Handle session start
      break;
    case 'user_speak':
      // Handle user speech
      break;
    // ... handle other events
  }
});

Response Requirements

All events (except session_end) accept a single action, an array of actions (executed in sequence), or 204 No Content:

• session_start: Can return action(s) or 204 No Content
• user_speak: Can return action(s) or 204 No Content (check barged_in flag for interruptions)
• dtmf_received: Can return action(s) or 204 No Content
• assistant_speak: Can return action(s) or 204 No Content
• assistant_speech_ended: Can return action(s) or 204 No Content
• user_input_timeout: Can return action(s) or 204 No Content
• session_end: No action allowed, cleanup only

Next Steps

• Session Start Event - Detailed reference
• User Speak Event - Detailed reference
• Action Types - How to respond to events