Vapi offers two main features that provide enhanced control over live calls:
  1. Call Control: This feature allows you to inject conversation elements dynamically during an ongoing call.
  2. Call Listen: This feature enables real-time audio data streaming using WebSocket connections.
To use these features, you first need to obtain the URLs specific to the live call. These URLs can be retrieved by triggering a /call endpoint, which returns the listenUrl and controlUrl within the monitor object.

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Obtaining URLs for Call Control and Listen

To initiate a call and retrieve the listenUrl and controlUrl, send a POST request to the /call endpoint.

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Sample Request

curl 'https://api.vapi.ai/call/phone' 
-H 'authorization: Bearer YOUR_API_KEY' 
-H 'content-type: application/json' 
--data-raw '{
  "assistantId": "5b0a4a08-133c-4146-9315-0984f8c6be80",
  "customer": {
    "number": "+12345678913"
  },
  "phoneNumberId": "42b4b25d-031e-4786-857f-63b346c9580f"
}'

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Sample Response

{
  "id": "7420f27a-30fd-4f49-a995-5549ae7cc00d",
  "assistantId": "5b0a4a08-133c-4146-9315-0984f8c6be80",
  "phoneNumberId": "42b4b25d-031e-4786-857f-63b346c9580f",
  "type": "outboundPhoneCall",
  "createdAt": "2024-09-10T11:14:12.339Z",
  "updatedAt": "2024-09-10T11:14:12.339Z",
  "orgId": "eb166faa-7145-46ef-8044-589b47ae3b56",
  "cost": 0,
  "customer": {
    "number": "+12345678913"
  },
  "status": "queued",
  "phoneCallProvider": "twilio",
  "phoneCallProviderId": "CA4c6793d069ef42f4ccad69a0957451ec",
  "phoneCallTransport": "pstn",
  "monitor": {
    "listenUrl": "wss://aws-us-west-2-production1-phone-call-websocket.vapi.ai/7420f27a-30fd-4f49-a995-5549ae7cc00d/transport",
    "controlUrl": "<https://aws-us-west-2-production1-phone-call-websocket.vapi.ai/7420f27a-30fd-4f49-a995-5549ae7cc00d/control>"
  }
}

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Call Control Feature

Once you have the controlUrl, you can inject a message into the live call using a POST request. This can be done by sending a JSON payload to the controlUrl.

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Example: Injecting a Message

curl -X POST 'https://aws-us-west-2-production1-phone-call-websocket.vapi.ai/7420f27a-30fd-4f49-a995-5549ae7cc00d/control' 
-H 'content-type: application/json' 
--data-raw '{
  "type": "say",
  "message": "Welcome to Vapi, this message was injected during the call."
}'
The message will be spoken in real-time during the ongoing call.

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Call Listen Feature

The listenUrl allows you to connect to a WebSocket and stream the audio data in real-time. You can either process the audio directly or save the binary data to analyze or replay later.

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Example: Saving Audio Data from a Live Call

Here is a simple implementation for saving the audio buffer from a live call using Node.js: 
const WebSocket = require('ws');
const fs = require('fs');

let pcmBuffer = Buffer.alloc(0);

const ws = new WebSocket("wss://aws-us-west-2-production1-phone-call-websocket.vapi.ai/7420f27a-30fd-4f49-a995-5549ae7cc00d/transport");

ws.on('open', () => console.log('WebSocket connection established'));

ws.on('message', (data, isBinary) => {
  if (isBinary) {
    pcmBuffer = Buffer.concat([pcmBuffer, data]);
    console.log(`Received PCM data, buffer size: ${pcmBuffer.length}`);
  } else {
    console.log('Received message:', JSON.parse(data.toString()));
  }
});

ws.on('close', () => {
  if (pcmBuffer.length > 0) {
    fs.writeFileSync('audio.pcm', pcmBuffer);
    console.log('Audio data saved to audio.pcm');
  }
});

ws.on('error', (error) => console.error('WebSocket error:', error));