code: def _input_accrued_national_number(self):
          
          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
docs: Invokes input_digit_helper on each digit of the national number
          accrued, and returns a formatted string in the end.