The plastic whistle
One of the prizes that came with Applebix in those days was a plastic whistle that had a tone similar to one used in the telecommunications industry as a signal to begin a particular computer process.
Some freaks had learned how to use it to enable free telephone calls, and the knowledge had spread.
Unfortunately for Cees, a small quantity of putty against the back of the chamber could make the whistle resonate at a frequency that would ring every telephone in the hotel ...
It was the only way that Adnoid could think of to alert his team that Piet was on the move.