Require every student to give his cell phone number. Then autodial until the culprit is found.
Frankly, I am appalled at the prevalence of cheating implied by so many of the threads on this subject. Any act of cheating should result in failure for the course, with the provision that it must be repeated with a C or better grade. Two acts of cheating is expulsion. Those were the conditions of the "Honor Code" at the schools I attended. It was actually a recruiting point in those halcyon days of the early 60's. I don't recall ever knowing first hand about someone cheating. Of all the years I was in school, I only knew from secondhand information of two cases that occurred.
I think cheating in universities and colleges is one example where deterrent is far more effective than detection.
John