'Invokes input_digit_helper on each digit of the national number
accrued, and returns a formatted string in the end.'

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
