Overrides the base class to add a PhoheNumberDescriptor rather than the standard FieldDescriptor

def add_to_class(self, cls, name):
        '''
        Overrides the base class to add a PhoheNumberDescriptor rather than the standard FieldDescriptor
        '''
        self.model_class = cls
        setattr(cls, name, PhoneNumberDescriptor(self))
        self._bound = True