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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def available_phone_numbers(self):
 """ Access the available_phone_numbers :returns: twilio.rest.api.v2010.account.available_phone_number.AvailablePhoneNumberCountryList :rtype: twilio.rest.api.v2010.account.available_phone_number.AvailablePhoneNumberCountryList """

        if self._available_phone_numbers is None:
            self._available_phone_numbers = AvailablePhoneNumberCountryList(
                self._version,
                account_sid=self._solution['sid'],
            )
        return self._available_phone_numbers