Brooklyn19,
If a kid wants to go off the derech having a cell phone may possibly make it easier for them to go off
However it is not (at least in the vast vast majority of cases) the reason why they want to go off and once they want to go off you not getting them a phone isnt going to change the situation they will do it anyway even if it is a little harder.
It would help alot more to change whatever it is that actualy makes kids want to go off instead of concentrating on things like cell phones wich dont.