Prevent Blocked Numbers from Calling Your Application

// Description
// Prevent blocked numbers from calling your application
// Host blocklist.json as a private Twilio asset

const fs = require('fs');

exports.handler = function (context, event, callback) {
  let twiml = new Twilio.twiml.VoiceResponse();

  // Phone numbers in blocklist.json file are rejected
  // blocklist.json is uploaded to Twilio Assets as a private asset
  // Format of blocklist.json is plain text:
  // ["+14075550100", "+18025550100"]

  let fileName = '/blocklist.json';
  let file = Runtime.getAssets()[fileName].path;
  let text = fs.readFileSync(file);
  let blocklist = JSON.parse(text);
  let blocked = true;

  if (blocklist.length > 0) {
    if (blocklist.indexOf(event.From) === -1) {
      blocked = false;
    }
  }

  if (blocked) {
    twiml.reject();
  } else {
    // if the caller's number is not blocked, redirect to your existing webhook
    twiml.redirect('https://demo.twilio.com/welcome/voice/');
  }

  return callback(null, twiml);
};