BBM. Maybe I'm remembering it wrong but didn't he say they had a fight or were arguing? That could be reason enough to turn it off, and perhaps the other phone in case he would try to reach her on that phone. It was late (10:30ish?) and had to be up early the next morning. As far as a phone being used as an alarm I think too much is being made about that. People often rely on that today but in 2006, not as much. She could shut it off and simply use on an alarm clock (remember those?).
But, if JK did turn both phones off that night, because of an argument with her BF, why did she not turn the phone on when she got up the next morning ?