title: Creating an android app with SMS based user registration process (a la Yo and WhatsApp)
body: <p>I am developing a free messaging app and I was looking to implement a user registration process based on the user's phone number and, for the verification of that number, I would like to implement a SMS mechanism similar to Yo, WhatsApp or Telegram apps, in which the user receives a unique confirmation code in an SMS to verify the phone number and activate the user's account. </p>
  
  <p>My research indicates that all these apps use third party SMS gateways providers like Twilio or Nexmo, for example, but that seems very costly for a free, and sort of recreational app, like Yo. So I wonder if free apps like Yo use paid SMS gateways providers to send confirmation codes to their users, or they indeed use anything else. </p>
  
  <p>I have also checked Telegram's Android app source code (<a href="https://github.com/DrKLO/Telegram" rel="nofollow">see here</a>) but I could not figure out how and what service they use to send SMSs with confirmation codes. Is anyone acquainted with Telegram's source code that could tell me how they do it?</p>
  
  <p>Much appreciated.</p>