title: Send push-notification from Android to iOS and vice versa?
body: <p>I've just learned that its going to be pretty tough for me to realize an app that can send a push notification to another device, BUT: is it even worse to send it from <strong>Android</strong> to <strong>iOS</strong> respectively <strong>iOS</strong> to <strong>Android</strong> ? I know it's possible because <code>WhatsApp</code> does it, but okay, WhatsApp... 
  The way I need it is, on Phone A <code>(Android)</code> a user clicks on a button "Send 'ring'" to user B <code>(iOS)</code>. 
  When A clicks that Button, an <code>AlertDialog</code> <code>(UIAlertView)</code> shall appear on user B's screen. 
  That's just to describe what I want to do, it obviously won't only send a 'ring'.</p>
  
  <p>Any suggestions or good tutorials?</p>