Twilio functions to forward phone calls to multiple cell phones. See https://cloudthingsyyc.wordpress.com/2018/01/15/hpaas-home-phone-as-a-service/ for more details.

incomingCalls.js

const format = require('string-format');

// These are the numbers to forward the call to
const destinations = ['+1', '+1'];

// These are the numbers that will be forwarded to the cell phones
const allowedCallers = [
  '+1', // 
];

// These are the numbers that will be hung up on right away
const blockedCallers = [
  '+1',
];

exports.handler = function(context, event, callback) {
  try {
    console.log(format('Incoming call: from={}, whitelist={}', event.From, JSON.stringify(allowedCallers)));
    
    let twiml = new Twilio.twiml.VoiceResponse();

    if (blockedCallers.indexOf(event.From) > -1) {
      twiml.hangup();
      callback(null, twiml);
      return;
    }

    if (allowedCallers.indexOf(event.From) > -1) {
      console.log('Forwarding call');
      let dial = twiml.dial();
	   
      // Set call destinations and filter     
      let numDest = destinations.length;
      for (var i = 0; i < numDest; i++) {
	let dest = destinations[i];
	// Make sure not to call the person who dialed in
	if (event.From !== dest) {
      	  dial.number(dest);
        }      
      }
    }
    else {
      console.log('Voicemail');
      twiml.say(
        'This is the phone of Mr. and Mrs. Smith. Please leave a message after the beep.\n' +
        'Thank you for calling!'
      );
      twiml.record({
        method: 'POST',
        maxLength: 120,
        transcribeCallback: 'https://yourdomain.twil.io/transcriptionComplete',
      });
      twiml.say('Unfortunately, I was unable to record the message, please call back again later.');
      twiml.hangup();
    }

    // return the TwiML
    console.log('Result: ' + twiml.toString());
    callback(null, twiml);
  } catch (ex) {
    callback('Failed to execute function: ' + JSON.stringify(ex));
  }
};

internationalCalls.js

const phoneRegistry = {
  1: {
    number: "+49123455667",
    name: "Max Mustermann"
  },
  2: {
    number: "+49123488998",
    name: "Maria Musterfrau"
  },
  // more numbers here
}

exports.handler = function(context, event, callback) {
  let twiml = new Twilio.twiml.VoiceResponse();
  
  // Ensure the incoming call is from one of the two whitelisted numbers.
  if (event.From !== "+14035555551" && event.From !== "+14035555552") {
    twiml.hangup();
    callback(null, twiml);
    return;
  }
  
  // If the selection was made during the previous function call, dial that number.
  let selection = event.Digits;
  if (selection) {
    twiml.dial(phoneRegistry[selection].number);
    
    callback(null, twiml);
    return;
  }

  let message = "Who would you like to call?";
  
  for (let key in phoneRegistry) {
    let entry = phoneRegistry[key];
    message += "Press " + key + " for " + entry.name + ".";
  }
  
  let gather = twiml.gather({
    numDigits: '1'
  });
  
  gather.say(message);
  
  // return the TwiML
  callback(null, twiml);
};

transcriptionComplete.js

const format = require('string-format')
const nodemailer = require('nodemailer');

exports.handler = function(context, event, callback) {
  try {
    console.log(format('Transcription result received: caller={}, status={}', event.From , event.TranscriptionStatus));  

    const transcript = event.TranscriptionStatus === 'completed' ?
      event.TranscriptionText :
      'No transcript!';

    let emailText = 'You received a voicemail from {from}. Transcription: \n\n{transcript}\n\nRecording: {recording}';

    console.log('Email template ready');
    sendEmail(
      event.From, 
      format(emailText, {
        from: event.From,
        transcript: transcript,
        recording: event.RecordingUrl
      }), 
      callback
    );
  } catch (ex) {
    callback('Failed to email transcription: ' + JSON.stringify(ex));
  }
};

/**
 * Send email message for the transcript and recording.
 *
 * @param {String} text
 */
function sendEmail(from, text, callback) {
  console.log('Sending email');

  let smtpConfig = {
    host: 'myhost.com',
    port: 465,
    secure: true,
    auth: {
      user: '[email protected]', 
      pass: 'somepassword',
    }
  };
  
  const transporter = nodemailer.createTransport(smtpConfig);

  const mailOptions = {
    from: '[email protected]',
    to: '[email protected]',
    subject: 'Voicemail left by ' + from,
    text: text,
  };

  transporter.sendMail(mailOptions, function(error, info) {
    if (error) {
      console.log('Failed to send email: ' + error);
      callback('Failed to send email: ' + error);
    } else {
      console.log('Email sent: ' + info.response);
      callback(null, null);
    }
  });
}