It must be pretty easy to block communications in a prison? Presumably it's a bit of a game, they want the prisoners communicating so they can track it??
Or install a StingRay / IMSI catcher at the prison and eavesdrop on the prisoners' calls. (Probably not legal, especially if you can't ensure you won't intercept calls originating from outside the prison)
Any time you run a StingRay in a populated area, you will grab some unintended calls. I'm under the impression (assuming judges are aware of this "collateral damage") that it's tolerable because if you are in the area you probably meet some threshold similar to "more likely than not" to have either committed a crime (or in the case of visitors to the prison, voluntarily given away your rights).
That said, I hope a tech-savvy judge gets to rule on a similar case eventually.