Max Masnick, PhD

July 23, 2017

How to Make an Anti-Spam Voicemail Number With Twilio Functions

SiriusXM keeps calling me because my car apparently came with a free trial, and I gave the dealer my cell number like a dummy. Now the trial is expiring, and they refuse to stop calling me.

There are lots of situations like this: you need to give someone a working phone number, and you most likely won’t want to hear from them. But in rare situations, you will want to – like if my car dealer has to contact me about a safety recall.

What I needed was a special phone number that would discourage telemarketers, but would allow a motivated person to contact me.

Services like Twilio and Plivo make it relatively easy to get a new phone number and allow it to receive calls and record voice messages. But until now there was too much overhead for this to be practical: in addition to the cloud phone service, I would need a separate web server to handle API requests.

This May, Twilio introduced “functions”, a feature that allows users to run custom server-side JavaScript, but without the server. This seemed like a perfect opportunity to create my an anti-spam voicemail number.

Here’s how to set up an anti-spam voicemail number with Twilio functions:

Step 1: Get a phone number

You can do that here for $1/month. You’ll need to set up a Twilio account if you don’t already have one.

Step 2: Set up a function to record a voicemail

Go here, click the + button, and name your function “Voicemail menu”. Set the path to be /voicemail-menu.

Here’s the code I used:

exports.handler = function(context, event, callback) {
  let actionUrl = 'https://some-subdomain.twil.io/send-sms'
  let twiml = new Twilio.twiml.VoiceResponse()
  switch (event.Digits) {
    case '7':
      twiml.say("Please leave a message after the beep. Hang up when finished.");
      twiml.record({
        'playBeep': true,
        'action': actionUrl
      });
      twiml.hangup();
      break
    default:
      twiml.gather({
          numDigits: 1
        })
        .say('Thank you for calling. If you would like to leave a voicemail, please press 7. Otherwise, you may hang up.')
  }
  callback(null, twiml)
};

This will read the following in a (pretty good) computer voice when someone calls my number:

Thank you for calling. If you would like to leave a voicemail, please press 7. Otherwise, you may hang up.

If they press 7, they will hear:

Please leave a message after the beep. Hang up when finished. BEEEEEEP

Otherwise, the call will disconnect.

Step 3: Set up a function to send you an SMS when you get a voicemail

Now you just need a way to get notified of the voicemal and listen to the recording. You can listen via Twilio’s dashboard, but it won’t notify you. My solution: send a text message to my cell phone with a link to the mp3 of the recorded voicemail.

Create a new function here named “Voicemail notifier via SMS” and set the path to be /send-sms.

Here’s the code I used:

exports.handler = function(context, event, callback) {
  twilioClient.messages.create({
    to: '+15555555555',
    from: event.To,
    body: `Voicemail from ${event.From || 'unknown'}: ${event.RecordingUrl || 'no recording'}`
  }, callback);
};

Replace +15555555555 with your cell number.

Step 4: Update the URL in Step 2

Grab the path from your “Voicemail notifier via SMS” function (it should look something like https://something-something-1000.twil.io/send-sms).

Then open up your “Voicemail menu” function and replace let actionUrl = 'https://some-subdomain.twil.io/send-sms' with let actionUrl = 'https://YOUR-SUBDOMAIN-GOES-HERE.twil.io/send-sms'.

Step 5: Attach your function to your phone number

Go here, click your new Twilio phone number, and under “Voice & Fax” choose:

Step 6: Test it out

Call your Twilio number from your phone. You should hear the prompt described above. If you press 7 and record a message, you should get a text message like this:

   
Example notification SMS. The message will be sent from your Twilio phone number. The blurred out phone number above will be the number for whoever left you the voicemail (in this case your own phone number).

Comments? Please send me a message.

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