Disable call waiting on Aastra 9133i


In our organisation we are using some Aastra 9133i SIP phones. I would like to disable the call waiting on some certain phones, so that they won’t get a second call while calling. It’s not possible on the webinterface of the phones so i suggest it must be done somewhere else. Where can i change this option? I don’t want it for all phones but just for some certain ones.

Thanks for your help