Is there any way to detect high tech (e.g. cell-phone assisted) academic dishonesty?
Ultimately, this is not a technological problem but a cultural and pedagogical problem.

The problem with technological solutions is that they set up an "arms race" with the students, in which you will certainly lose.  Electronics are constantly getting smaller and obtaining more means of communication, so a student who really wants to cheat will likely have a means that you won't be readily able to distinguish.  Consider, for example, the recent emergence of smart watches: are you going to ban watches from the classroom?  What about low-tech but still effective forms of cheating like scraps of paper?  If you get a frequency scanner, will you be able to catch somebody using non-standard communication bands?

The best defense against cheating students is other students who understand that it cheats them out of the value of their well-earned grades.  If you can inculcate a school culture where most students are not just not cheating but actively opposed to cheating, then it will be much harder for cheaters to prosper.

The second best defense against cheating is to design exams that are more about process than product.  Think "show your work" and "essay question."  Yes, it's possible that a cheating student will outsource their work and render themselves a puppet of an expert whispering guidance in their ear, but that's a lot harder to do than secretly googling for information about Kirchoff's laws.  I am, in fact, a big fan of open-book tests, which tend to push students to focus on synthesis instead of memorization. This, I believe, has much more value to students in the long run, and also has the nice side effect of rendering high-tech cheating much less valuable as well.