Seriously? To stop cheating?
Whatever happened to the simple expedient of having invigilators walking around the exam rooms. Anyone caught with a mobile phone in hand is kicked out and fails. Put enough invigilators in the room and cheating will not be a problem. It's what used to be done at my university, (i assume it still is) along with providing the calculators needed for exams (preventing programmed calculators doing the actual work). Even if someone smuggles a phone in, with people walking around with the power to kick them out and fail them, that phone will stay resolutely in that pocket. Guaranteed.
It's really not a difficult problem to solve and you surely dont need to cripple your economy doing so.
(Ok maybe these nations dont have the physical cash in hand to pay people to do invigilating, but surely they can frame it as some sort of civic duty, or put the military to work. They dont have anything else to do, right? :P)