Call Control API

The Call Control API provides programmatic control over active calls. Use it to play TTS, hang up, transfer calls, send DTMF, or put calls on hold — all via RESTful HTTP/JSON endpoints. This is the imperative counterpart to the declarative dialog FSM.

Base URL

All endpoints are relative to your voicetyped host:

https://gateway.example.com/v1/calls/{id}/...

Replace {id} with the active call session ID. All request and response bodies use JSON with Content-Type: application/json.

Authentication

Include your API token in the Authorization header:

Authorization: Bearer <token>

Endpoints

Play TTS

Render text to speech and play it on an active call.

POST /v1/calls/{id}/tts

Request body:

Field	Type	Required	Description
`text`	string	yes	Text to speak
`voice`	string	no	Voice model (default: server-configured)
`speed`	float	no	Playback speed (default `1.0`)
`barge_in`	boolean	no	Allow caller to interrupt (default `true`)

Response body:

Field	Type	Description
`playback_id`	string	ID to track this playback
`estimated_duration_ms`	integer	Estimated playback duration in milliseconds

Example:

curl -X POST https://gateway.example.com/v1/calls/call-abc-123/tts \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "text": "Your ticket has been created. The ticket number is 45678.",
    "voice": "en_US-amy-medium",
    "speed": 1.0,
    "barge_in": true
  }'

{
  "playback_id": "pb-9f2a-4e71",
  "estimated_duration_ms": 3200
}

Hangup

Terminate an active call.

POST /v1/calls/{id}/hangup

Request body:

Field	Type	Required	Description
`reason`	string	no	Reason for hangup: `normal`, `busy`, `rejected`, or `error` (default `normal`)

Response body:

Field	Type	Description
`call_duration_seconds`	integer	Total call duration in seconds

Example:

curl -X POST https://gateway.example.com/v1/calls/call-abc-123/hangup \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "reason": "normal"
  }'

{
  "call_duration_seconds": 142
}

Transfer

Transfer an active call to another SIP endpoint.

POST /v1/calls/{id}/transfer

Request body:

Field	Type	Required	Description
`target_uri`	string	yes	SIP URI to transfer to
`type`	string	no	Transfer type: `blind` or `attended` (default `blind`)
`headers`	object	no	Custom SIP headers as key-value pairs

Response body:

Field	Type	Description
`success`	boolean	Whether the transfer was initiated
`transfer_id`	string	ID to track this transfer

The call is immediately redirected to the target URI:

curl -X POST https://gateway.example.com/v1/calls/call-abc-123/transfer \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "target_uri": "sip:support@pbx.internal",
    "type": "blind",
    "headers": {
      "X-Transfer-Reason": "escalation",
      "X-Original-Caller": "+15551234567"
    }
  }'

{
  "success": true,
  "transfer_id": "xfer-7c3d-88a1"
}

Attended Transfer

The system first establishes a call to the target, then bridges the caller:

curl -X POST https://gateway.example.com/v1/calls/call-abc-123/transfer \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "target_uri": "sip:agent@pbx.internal",
    "type": "attended",
    "headers": {}
  }'

Send DTMF

Send DTMF digits on an active call. Useful for navigating external IVR systems.

POST /v1/calls/{id}/dtmf

Request body:

Field	Type	Required	Description
`digits`	string	yes	Digits to send (`0`-`9`, `*`, `#`)
`duration_ms`	integer	no	Tone duration per digit in milliseconds (default `100`)
`gap_ms`	integer	no	Gap between digits in milliseconds (default `50`)

Example:

curl -X POST https://gateway.example.com/v1/calls/call-abc-123/dtmf \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "digits": "12345",
    "duration_ms": 100,
    "gap_ms": 50
  }'

{
  "success": true
}

Hold

Put an active call on hold.

POST /v1/calls/{id}/hold

Request body:

Field	Type	Required	Description
`music_file`	string	no	Path to hold music file (WAV, MP3)

Example:

curl -X POST https://gateway.example.com/v1/calls/call-abc-123/hold \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "music_file": "/path/to/music.wav"
  }'

{
  "success": true
}

Resume

Resume a held call.

POST /v1/calls/{id}/resume

Request body:

Empty object or omitted.

Example:

curl -X POST https://gateway.example.com/v1/calls/call-abc-123/resume \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{}'

{
  "success": true
}

Play Audio

Play a pre-recorded audio file on an active call.

POST /v1/calls/{id}/play-audio

Request body:

Field	Type	Required	Description
`file_path`	string	yes	Path to audio file (WAV, MP3)
`loop`	boolean	no	Loop playback (default `false`)
`barge_in`	boolean	no	Allow caller to interrupt (default `true`)

Response body:

Field	Type	Description
`playback_id`	string	ID to track this playback

Example:

curl -X POST https://gateway.example.com/v1/calls/call-abc-123/play-audio \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "file_path": "/var/audio/greeting.wav",
    "loop": false,
    "barge_in": true
  }'

{
  "playback_id": "pb-4a8e-c312"
}

Stop Playback

Stop any currently playing audio or TTS.

POST /v1/calls/{id}/stop-playback

Request body:

Field	Type	Required	Description
`playback_id`	string	no	Stop a specific playback by ID. If omitted, stops all active playback on the call.

Example:

curl -X POST https://gateway.example.com/v1/calls/call-abc-123/stop-playback \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "playback_id": "pb-9f2a-4e71"
  }'

{
  "success": true
}

Error Handling

All endpoints return standard HTTP status codes:

Code	Meaning
`200 OK`	Success
`404 Not Found`	Session ID does not exist or call has ended
`409 Conflict`	Call is in wrong state (e.g., already on hold)
`400 Bad Request`	Missing or malformed parameters
`503 Service Unavailable`	Service is temporarily unavailable
`500 Internal Server Error`	Unexpected error

Error responses include a JSON body with details:

{
  "error": {
    "code": 404,
    "status": "NOT_FOUND",
    "message": "Call session call-abc-123 not found or has already ended."
  }
}

Usage Pattern

A common pattern is combining the Call Event Stream API with the Call Control API for reactive call management. Subscribe to a server-sent event stream for real-time call events, then issue control commands in response.

#!/usr/bin/env bash

TOKEN="your-api-token"
BASE="https://gateway.example.com"

# 1. Subscribe to speech events via SSE (runs in the background)
curl -N -H "Authorization: Bearer $TOKEN" \
  "$BASE/v1/events?types=speech_final" | while read -r line; do

  # Parse the event data (assumes each line is a JSON event)
  session_id=$(echo "$line" | jq -r '.session_id // empty')
  transcript=$(echo "$line" | jq -r '.speech.transcript // empty')

  [ -z "$session_id" ] && continue

  # 2. If the caller says "transfer", announce and transfer
  if echo "$transcript" | grep -qi "transfer"; then
    # Play a TTS announcement
    curl -s -X POST "$BASE/v1/calls/$session_id/tts" \
      -H "Authorization: Bearer $TOKEN" \
      -H "Content-Type: application/json" \
      -d '{
        "text": "Transferring you now. Please hold.",
        "barge_in": false
      }'

    # Wait briefly for the announcement, then transfer
    sleep 3

    curl -s -X POST "$BASE/v1/calls/$session_id/transfer" \
      -H "Authorization: Bearer $TOKEN" \
      -H "Content-Type: application/json" \
      -d '{
        "target_uri": "sip:agent@pbx.internal",
        "type": "blind",
        "headers": {}
      }'
  fi
done

For production use, consider implementing this pattern in your application language with a proper HTTP client and SSE parser rather than shell scripting.

Next Steps

Call Event Stream API — subscribe to call events
Dialog Hooks API — implement backend integration
Speech API — direct ASR access

Call Control API

Base URL

Authentication

Endpoints

Play TTS

Hangup

Transfer

Blind Transfer

Attended Transfer

Send DTMF

Hold

Resume

Play Audio

Stop Playback

Error Handling

Usage Pattern

Next Steps