From my humble point of view, there's any number of other methods pf communications they can use, and to be honest, if they've managed to get on the flight, it's probably already too late.
As at the minute I'm humble SLF (but working through my ATPL exams) I enjoy the fact that mobiles are not allowed - travel on trains regularly, and you'll know how annoying it is everytime someone's latest crap ringtone chimes through the air, followed by "Hello... yeah, I'm on the train..."