Handle an incoming call with Node.js SDK

Now that you know how to call yourself using the Voice API, learn how to handle incoming calls.

In this guide you will learn:

1. How to set up your Node.js application
2. How to start your web server and set up a tunnel
3. About callbacks
4. How to call your server

Set up your Node.js application

First we'll create a Node project using npm. This creates a package.json and the core dependencies necessary to start coding.

To create the project, do the following steps:

1. Create a folder called receive-sms-app
2. Navigate into the folder you created and run the following command.
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   npm init

   This command adds the package.json file. You will be prompted to provide values for the fields. For this tutorial, you can simply accept the default values and press enter at each stage.

   You can install the Sinch Node.js SDK using either NPM or Yarn:

   NPMYarn

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   npm install @sinch/sdk-core

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   yarn add @sinch/sdk-core

3. Next, install the Fastify package. Fastify is a lightweight webserver that we will use to respond to webhooks.
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   npm install fastify

import { SinchClient } from '@sinch/sdk-core';
import Fastify from 'fastify';

Create your file

Save your file.

Start your web server and set up a tunnel

1. Start the server by executing the following command:
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   node index.js

2. Open a tunnel to the server you just set up. We are using ngrok for this. If you don't have ngrok installed already, install it with the following command:
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   npm install ngrok -g

3. Open a terminal or command prompt and enter:
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   ngrok http 3000

4. Navigate to your app on your dashboard. Under the Settings section, you'll see a field labeled "Callback URL." Enter your URL into that field and click Save.

Now your server is listening and your callback URL is configured, so you're almost ready to test everything and make a phone call. But before we do, let's take a closer look at callbacks. If you already know about callbacks, skip right to calling your Sinch phone number.

Understanding callbacks

Callbacks (also known as "webhooks") are the method that the Voice API uses to figure out what you want to do with a call. Basically, a callback is a request that the Sinch servers send to your server whenever something happens (otherwise known as an "event") in the call that requires some input from you. There are a few different types of events, but the two we are concerned about for this guide are the Incoming Call Event and the Disconnected Call Event.

An Incoming Call Event happens whenever someone calls one of your Sinch numbers. In essence, someone dials your number and so Sinch servers reach out to you and say "how do you want me to handle this call?"

Most callback events expect a response, depending on the event. The Incoming Call Event expects to receive back a SVAML object in response. You can read more about SVAML here, but just know that SVAML is a markup language Sinch developed to control calls.

The below diagram demonstrates exactly what's happening:

1. Someone dials your Sinch number from a handset.
2. The Sinch servers create an ICE and send a POST request to your server.
3. Your server listens for the request and sends back a SVAML response to tell the Sinch server how to handle the call.

In this sample application, this is the SVAML object that you will use to respond to the callback (we've also provided the underlying JSON SVAML object for comparison):

const iceResponse = new Voice.IceSvamletBuilder()
        .setAction(Voice.iceActionHelper.hangup())
        .addInstruction(Voice.iceInstructionHelper.say('Thank you for calling Sinch! This call will now end.', 'en-US'))
        .build();

{
  "instructions": [
    {
      "name": "say",
      "text": "Thank you for calling Sinch! This call will now end..",
      "local": "en-US"
    }
  ],
  "action": {
    "name": "hangup"
  }
}

This SVAML object has two parts: instructions and an action. Instructions are things you want to be done on the call without changing the state of the call. In this case, we want to play a voice that reads out a text message. Actions are things you want to be done to the call to change its state in some way. In this case, we want to hang up and end the call.

And that's it! Now we can test.

Call your Sinch phone number

Look up the free Sinch number assigned to your app and call it using your phone. The call should be picked up by the Sinch servers and you should hear the text from the instruction. Now you know how to handle an incoming call.

Next steps

Learn more about the Voice API:

• API specification