You would need to allow or open a port in you router to steer or "forward" the request to the ip address of you Arduino. Also if your using port 80 your ISP most likely blocks requests to port 80 so if you reassign it to port 8080 that should work. You will need to know your ip address assigned to your router you could use whatsmyip google it. Then enter http://youripaddress:8080
on your phone and if all is well it work.
Thanks for your reply. It seems that this still doesn't allow me to access my arduino server from outside my wifi network...