FMGC In a modern ATC system the controller's display shows a difference between "own" and "other" traffic. If a flight entering a sector has not called on the frequency this is immediately obvious to the controller and after calling the flight a couple of times his next action is to contact the previous controller who will then call the flight to check if he has not QSYed for any reason.
For some reason there seem to be periods when it is always the same operator whose flights are not calling. Some years ago we had Air France doing it on an almost daily basis; then for a while it was SAS.
It sometimes seemed that the only solution would be to refuse the flights as their equipment/crews were not up to standard required. Fortunately I don't think it ever got that far