Create a webhook server

When an inbound call is received, Vonage makes a request to a publicly accessible URL of your choice - we call this the answer_url. You need to create a webhook server that is capable of receiving this request and returning an NCCO containing a connect action that will forward the call to the PSTN phone number. You do this by extracting the destination number from the to query parameter and returning it in your response.

New project

Create a new project directory in a destination of your choice and change into it:

mkdir app-to-app-kotlin
cd app-to-app-kotlin

Inside the folder, initialize a new Node.js project by running this command:

npm init -y

Add dependencies

Next, install the required dependencies:

npm install express localtunnel --save

Create the server file

Inside your project folder, create a file named server.js and add the code as shown below - please make sure to replace SUBDOMAIN with an actual value. The value used will become part of the URLs you will set as webhooks in the next step.

'use strict';

const subdomain = 'SUBDOMAIN';

const express = require('express')
const app = express();

app.get('/voice/answer', (req, res) => {
  console.log('NCCO request:');
  console.log(`  - caller: ${req.query.from}`);
  console.log(`  - callee: ${req.query.to}`);
  var ncco = [{"action": "talk", "text": "No destination user - hanging up"}];
  var username = req.query.to;
  if (username) {
    ncco = [
        "action": "talk",
        "text": "Connecting you to " + username
        "action": "connect",
        "endpoint": [
            "type": "app",
            "user": username

app.all('/voice/event', (req, res) => {

if(subdomain == "SUBDOMAIN") {
  console.log('\n\t🚨🚨🚨 Please change the SUBDOMAIN value');
  return false;

const localtunnel = require('localtunnel');
(async () => {
  const tunnel = await localtunnel({ 
      subdomain: subdomain, 
      port: 3000
  console.log(`App available at: ${tunnel.url}`);

NOTE: Please remember to replace SUBDOMAIN with a random string of your choice, containing letters, numbers, underscores or dashes.

There are 2 parts in the server code above:

The Express server

The first part creates an Express server and makes it available locally on port 3000. The server exposes 2 paths:

  1. /voice/answer is the answer_url we mentioned above. It sends back a JSON response containing the destination number for the call.

    Notice, that the number is extracted from the req.query.to parameter that Vonage is sending as part of the request. The dynamically built NCCO then forwards the call to the destination phone using a connect action.

  2. The second one, /voice/event, you will set as destination for Vonage to notify you of everything happening during the call - - we call this the event_url.

The localtunnel integration

The second part of the server code above, exposes the Express server so it will be accessible by the Vonage servers.

NOTE: localtunnel is a JavaScript library that exposes your localhost to the world for painless testing and sharing! No need to mess with DNS or deploy to have others test out your changes.

Start the server

You can now start the server by running, in the terminal, the following command:

node server.js

A notice will be displayed telling you the server is now available:

App available at: https://SUBDOMAIN.loca.lt

Please keep the terminal window handy as you will need the URL in the next step.