Thousands of prisoners have cell phones, often buying them from the guards or having visitors smuggle them in. And cell phones are even tossed or shot over prison walls (which makes me wonder what else is getting delivered the same way - seems like guns or drugs would lend themselves to the same methods.)
And while cell phones might be used for benign purposes, like staying in touch with family, they also allow criminals to organize crimes while behind prison walls.
Read more here.