code: def calls(self):
          
          if self._calls is None:
              self._calls = CallList(self._version, account_sid=self._solution['sid'], )
          return self._calls
docs: Access the calls
  
          :returns: twilio.rest.api.v2010.account.call.CallList
          :rtype: twilio.rest.api.v2010.account.call.CallList