Noise-cancelling headsets are known to cancel out warnings and cautions.
By whom? There is a vast amount of disinformation (like that whopper of a quote above) floating around on forums like this about ANR headsets by people who know not the first thing about how they work or what they do. They do work. You can do as I did, send an email to Bose and be chatting on the telephone with one of their engineers in 48 hours if you really want to know.
Never flown a medium or larger turbine helicopter have you, Belly? Or a good sized turboprop, eh?
David Clark makes very fine ANR headsets and very fine conventional headsets. Bose also makes very fine ANR headsets. Service for both is exceptionally good in the U.S., can't speak to how good it is overseas. The upfront cost is significant for either one, but if you plan on doing this for a living, worth the investment to save your ears and to be able to hear your grandkids one day.