No, the batter is not allowed to hold a cell phone while batting.
Actually, they are banned in the dugout.
There are some really, really tiny ones that prison inmates get smuggled in from time to time................inside.......orifices.........
I would bet that a lot of personal cheating happens by device now. Just put a small button buzzer in a sock or under an athletic wrap that triggers when someone looking at a feed of the catcher thinks an off speed pitch is coming.
Freegards