title: Services (such as Twilo, Plivo, Nexmo) to verify phone numbers in Rails app
body: <p>I'd like to verify phone numbers of user accounts in my Rails 4 app (by simply sending them a four digit pin number which they'll need to enter back into the app) - which services are available and which ones are the least hassle to implement into a Rails app?</p>
  
  <p>I'd like to verify both mobile and landline numbers internationally, though my main areas will be Europe, the US &amp; Canada, Australia and New Zealand (I am based in the UK).</p>