code: def voip(self):
          
          if self._voip is None:
              self._voip = VoipList(
                  self._version,
                  account_sid=self._solution['account_sid'],
                  country_code=self._solution['country_code'],
              )
          return self._voip
docs: Access the voip
  
          :returns: twilio.rest.api.v2010.account.available_phone_number.voip.VoipList
          :rtype: twilio.rest.api.v2010.account.available_phone_number.voip.VoipList