My approach is simple, effective and "fun". On day one I explain my cell phone policy. If they are expecting an "emergency call" they can have the phone on vibrate and quitely exit the classroom to take the call. Otherwise, if thier cell phone goes off or if they are caught texting, anyone in the classroom can yell "treat" and that student is responsible for bringing something in for everyone the next class period. Of course cost is a consideration and something like a piece of inexpensive candy, a piece of gum etc. is acceptable.