Building a Voice Alerts Broadcast System using Sinatra and the Nexmo Text-to-Speech APIJune 21st, 2016Cristiano Betta7 minute read

This post was originally first posted on the Nexmo Developer Blog on June 7th 2016.

Sometimes a text message won't do. When you really want to be alerted of something important you probably want to receive an actual phone call, wether it's because a server is down, or because a natural disaster has occurred in your area.

Similarly as a service provider or (local) authority dealing with an emergency I want to reach out to as many people as possible, as quickly as possible, regardless if they own a mobile phone.

The flood alerts app

For this tutorial I am going to show you how to build a Voice Alerts Broadcast System using the Nexmo Text-To-Speech API (TTS). For this purpose I have built my own Nexmo Flood Alerts service that allows people to subscribe to voice alerts for their postcode.

Nexmo Flood Alerts

You can download the starting point of the app from Github and run it locally.

# ensure you have Ruby and Bundler installed
git clone
cd nexmo-sinatra-voice-alerts-demo
bundle install
bundle exec rake db:migrate
ruby app.rb

Then visit localhost:4567 in your browser and subscribe to a flood alert with your postcode and your number. After this you can visit the (very rudimentary) admin panel where you should be able to see your subscription.

The app is a basic Sinatra app, written in Ruby with a few extra gems to make things easier. I've added SQLite3 and ActiveRecord for persisting our Subscribers. I also added Bootstrap to make our UI look a bit more attractive. For this tutorial I've left the fetching of the flood data as an implementation detail and instead I will be sending the notifications by hand through the admin panel.

All the code for this starting point can be found on the subscriptions branch on Github. All the code I will be adding below can be found on the alerts branch. For your convenience you can see all the changes between our start and end point on Github as well.

Nexmo Text-To-Speech

Nexmo Voice is the easiest way to build high quality voice applications in the cloud. It has plenty of features and the one we will be looking at today is the Text-To-Speech API (TTS). We will use this API to send people voice calls that will speak out the flood alerts to them. To do this we are going to add the following changes to our app:

  1. Add Nexmo to our app
  2. Create a form to allow the admin to select who receives the alerts
  3. Send a request to the Nexmo API for a Text-To-Speech message

Adding Nexmo to our app

In order to send a Text-To-Speech message via Nexmo I am going to have to add the nexmo gem to my project.

# Gemfile
gem 'nexmo'
gem 'dotenv'

As you can see I also added the dotenv gem. This is so that the app can load my API credentials from a .env file. The Nexmo gem automatically picks up those environment variables and uses them to initialize the client. You can find your credentials on the settings page of your Nexmo account.

# .env

We also need to tell our app to use these two gems on start.

# app.rb
require 'nexmo'
require 'dotenv'

Now that we've loaded these 2 gems we can simply call without any parameters and use this in our app anywhere we want.

Select a number or postcode

Alert Form

Before we start sending alerts to everyone we should probably give the admin a way to select who to send a message to. Let's start by adding a form to the admin panel.

<!-- views/admin.erb -->

<h2 class='header'>Send an alert</h2>

<form class="form form-vertical" action="/alert" method="post">
  <div class="form-group form-inline">
    <select class="form-control" name="number">
      <option disabled selected value> -- select a number -- </option>
      <% Subscriber.pluck(:number).sort.each do |number| %>
        <option><%= number %></option>
      <% end %>
  <input type='submit' value='Send' class='btn btn-primary'>

Ignoring the Bootstrap boilerplate, all we did here is to create a form with 1 field that gives us the option to target a phone number. Let's add a second field though that let's us choose from all the postcodes as well.

<!-- views/admin.erb -->
<select class="form-control" name="postcode">
  <option disabled selected value> -- select a postcode -- </option>
  <% Subscriber.pluck(:postcode).sort.each do |postcode| %>
    <option><%= postcode %></option>
  <% end %>

Send a Text-To-Speech message

When the admin submits this form it will POST a request to /alert where we can parse the number or postcode selected and make a call to the Nexmo API.

# app.rb
post '/alert' do
  if params[:number]
    send_alert(:number, params[:number])
  elsif params[:postcode]
    send_alert(:postcode, params[:postcode])
  redirect '/alert'

To actually send the message we will pass along 4 parameters to the initiate_tts_call method of the Nexmo::Client.

  • to - The number to send a voice message to.
  • from - The Nexmo Voice number to send a voice message from
  • message - The message to speak out to the recipient
  • lg - The voice language to use. In this case we will use a British voice.

To get a Nexmo Voice Number either head over to the Nexmo Dashboard and purchase a number, or alternatively use the Nexmo CLI and buy a number right on the command line.

With this number purchased we can now send our actual message.

# app.rb
def send_alert key, value
  Subscriber.where(key => value).each do |subscriber|
        to: subscriber.number,
      from: ENV['NEXMO_PHONE_NUMBER'],
      text: %{
        <break time="1s"/> Hello #{}.
        This is a flood alert for
        <prosody rate="-50%">#{subscriber.postcode}</prosody>.
        Thank you for using Nexmo.
        lg: 'en-gb'

Here we find all the Subscribers that match the given postcode or number, and then for each we make a call to the Nexmo API. In my app I put my number in the .env file (NEXMO_PHONE_NUMBER) and you can do the same thing or you could just put it directly into your code above.

The voice message's text parameter has a few interesting things embedded in the actual text. First off I used the <break/>-tag to pause for a second at the start of the call to wait for the recipient to put the phone to their ear. Similarly I used the <prosody>-tag to slow down the pronunciation of the postcode. More about these tags can be found in our documentation.

Finally all we need to do is make sure that when the admin is redirected they see a page that confirms they sent the alert.

# app.rb
get '/alert' do
  erb :alert
<!-- views/alert.erb  -->
<% content_for :title do %>
  Alert sent
<% end %>

  Your Nexmo Flood Alert has been sent.

That's it, give it a try! Make sure to restart Sinatra if needed. Select your own number from the dropdown and submit the form. You should receive a voice call within seconds!

Next steps

The Nexmo Text-To-Speech API has a lot more options than I showed here, and I actually skipped a few very powerful ones:

  • Nexmo can automatically retry a call for you up to 10 times
  • Nexmo can automatically detect answering machines and either leave a message or hangup
  • You can specify a webhook endpoint for Nexmo to call after an alert has been successfully received
  • You can specify the preferred gender of the voice used to turn your text into speech

Personally I really like it that I can send a Text-To-Speech message like this without directly needing webhooks, allowing me to add webhooks only if needed or on a later date.

I'd love to know what you'd add next? Leave me your ideas in the comments below.