"You are beautiful and wonderful - keep going! (unlike this systems security)"
Which incidentally, how do you enforce a rule like this without serious security like pat downs or metal detectors? Because otherwise you could easily sneak a phone in through the sophisticated hack of owning two phones…
And as for enforcement, yeah there isn't an effort in trying to avoid anyone sneaking in an extra phone, it's basically grounded on good faith and the fact that if they catch your phone with you (or even if it rings in the bag), you're eliminated from the exam. That's where I think the chaos would arise. If everyone's phones alerted at the same time, the examiner wouldn't know what to do.
"This is Army Commander Tomás Miguel Ribeiro Paiva. We have chosen to take command of the country to protect you against serious crimes against the people that we have become aware of. Remain calm and continue with your daily duties."
(Except in Brazilian Portuguese.)
I guess so scary that there isn't a single person willing to try it. But yeah, that is the most dangerous one possible.