Or, as the Cell Phone Lock Up's sellers envision, torture your friends with the thing: "You'll really get 'em when you lock up their precious cell phone in this confounding wooden contraption! Watch as they flip, shake and even beg as they try to free their phone." This situation sounds really pathetic and uncomfortable and not very prank-like. Mostly just sad.
The site doesn't explain how the trap actually locks - is there a key? Magnets? But it could be the niche item the smartphone sadomasochist in your life is missing. [Cell Phone Lock Up via Random Good Stuff]