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) {
  const 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"]
  const fileName = '/blocklist.json';
  const file = Runtime.getAssets()[fileName].path;
  const text = fs.readFileSync(file);
  const blocklist = JSON.parse(text);
  
  const blocked = blocklist.length > 0 && blocklist.includes(event.From);

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

  return callback(null, twiml);
};