docs: 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