title: login to WordPress with phone number instead of username
body: <p>I intend to write a WordPress plugin that enable users to log in using mobile phone number and password instead of username. The first time the user will register with phone number and password and the plugin will send a confirmation code to them, and after that the user will login using the phone number and password. Can I use phone number instead of username by preventing the user from entering any characters other than numbers in the WordPress registeration form? and is this the best way to do that?
  Any help would be greatly appreciated.</p>