def _input_accrued_national_number(self):
        """Invokes input_digit_helper on each digit of the national number
        accrued, and returns a formatted string in the end."""
        length_of_national_number = len(self._national_number)
        if length_of_national_number > 0:
            temp_national_number = U_EMPTY_STRING
            for ii in range(length_of_national_number):
                temp_national_number = self._input_digit_helper(self._national_number[ii])
            if self._able_to_format:
                return self._append_national_number(temp_national_number)
            else:
                return self._accrued_input
        else:
            return self._prefix_before_national_number
Invokes input_digit_helper on each digit of the national number
        accrued, and returns a formatted string in the end.