Dad's dead, mom is in a war zone. It's difficult to get to call home from a war zone - I knew a girl who, whenever her boyfriend called home from Iraq, dropped whatever they were doing to answer the call because of that. Chances for phone calls are few and far between, and really expensive.
It's all about discretion. Under those circumstances, if I were the teacher, I'd let him bring the cell phone into class, as long as it didn't disrupt (put it on vibrate, etc.) He was out on his lunch break, talking with a mother overseas fighting for our country in an unpopular war, causing no one any trouble. The teacher walks up and tries to take his phone from him? Hell no. That teacher is lucky she didn't get a punch in the face - because she deserves it.
And considering the very real danger our soldiers are in every day, any conversation he has with her could very well end up being the last one ever. Don't lose sight of that.