The idea (although its lost in translation through the poor wording of the NOTAM) is NOT to call at all, and to remain completely silent until you are called by ATC.
As ATCOs we have an indication in the radar label of when at aircraft has been handed over internally via the 'standby' protocol.
It would have been nicer to use 'monitor' in the NOTAM, as that is pretty much self-explanatory - however it was not possible.
If, as a pilot, you are told to 'standby for Maastricht', then switch on the box but do NOT call in.