code: def assigned_add_ons(self):
          
          if self._assigned_add_ons is None:
              self._assigned_add_ons = AssignedAddOnList(
                  self._version,
                  account_sid=self._solution['account_sid'],
                  resource_sid=self._solution['sid'],
              )
          return self._assigned_add_ons
docs: Access the assigned_add_ons
  
          :returns: twilio.rest.api.v2010.account.incoming_phone_number.assigned_add_on.AssignedAddOnList
          :rtype: twilio.rest.api.v2010.account.incoming_phone_number.assigned_add_on.AssignedAddOnList