@staticmethod
def __format_phone_number(phone_number):
    """
        Format a local Vietnamese phone number to its international
        representation.

        The function adds a leading digit `0` if the argument `phone_number`
        is only composed of 9 digits.


        :param phone_number: A local Vietnamese phone number composed of 9 or
            10 digits.


        :return: A string representation of an international phone number,
            starting with `+84.`.


        :raise ValueError: If the argument `phone_number` is not composed of
            9 or 10 digits only.
        """
    if not phone_number.isdigit():
        raise ValueError(f'invalid phone number {phone_number}')
    if len(phone_number) < 9:
        raise ValueError(f'the phone number {phone_number} is missing digits')
    return f'+84.{phone_number.zfill(10)}'
