+isRegistered() -> result<b>::
+isRegistered(number<s>) -> result<b>::
+isRegistered(numbers<as>) -> results<ab>::
+* number : Phone number
+* numbers : String array of phone numbers
+* result : true=number is registered, false=number is not registered
+* results : Boolean array of results
+
+Exception: InvalidNumber for an incorrectly formatted phone number. For unknown numbers, false is returned, but no exception is raised. If no number is given, returns whether you are registered (presumably true).