Only one or two corps have that rule. That's a bad excuse to not try out for a corps. My two most recent summers: One where I talked on the cell phone every night, one where I wasn't allowed to have one. As a result, I made no new friends in corps #1 (I knew some guys going in though), and I made many many new friends in corps #2. And if you really need to, it's easy to borrow one from any of the 30ish staff/volunteers/junior officers whenever you want. No cell phone was one of the best things I ever did.