Taking my own phone into a hostile situation on the other hand, why would you? I don’t even take my own phone when travelling for business.
Huh? That makes absolutely no sense to me. Being on business travel is exactly when I want my phone.
Maybe just reuse the phone number?
Get a similar soft display case with compartments big enough to hold cell phones, and have it displayed at the front of the class where everyone can see them. You put your phone in one of the “pouches,” you can see it the whole time you’re taking an exam, you pick it back out once you’re done.
I’m not sure what a practical method of ensuring you get the right phone back would be, but I can think of a few schemes. For example, each pouch has a uniquely numbered card, and when you put your phone in there, you take the card. Then you put the card back when you pick your phone back up. It would probably be helpful to have 1 or 2 visual indicators that you got it right, like the coordinates from the card also printed on the clear plastic in a way that’s easy to see them match, plus maybe a color scheme where a stripe or sticker of some kind on the clear plastic matches the color of the card itself. (This all to prevent accidental mix ups, not theft. The main benefit I see is letting the people still taking the exam feel confident that their phone is visible to them and it’ll remain where it is until they pick it up.)
Those are objective risks. Then you should also factor in subjective belief that when the phone is in your hands then you control its safety, while when it's out of your hands then anything can happen. When you're not in control, you tend to get pretty anxious.
Then you can add in other nightmares. What if the screen gets scratched? What if the phone gets crushed? What if someone takes your phone and accidentally drops it? What if they take it and leave with it? What if someone spills their coffee into the box? What if someone touches your phone with their filthy greasy hands and you get corona because of that?
Mildly worried about both on purpose and by accident.
(My GP comment was meant ironically, like, wouldn't this be crazy if: ...)