def add_contact(self, phone_number: str, first_name: str, last_name: str=None, on_success: callable=None):
        """
        Add contact by phone number and name (last_name is optional).
        :param phone: Valid phone number for contact.
        :param first_name: First name to use.
        :param last_name: Last name to use. Optional.
        :param on_success: Callback to call when adding, will contain success status and the current contact list.
        """
        pass