Not sure what you are saying. If you mean that a jammer isn't tunable, and blasts out energy at all frequencies, you are wrong. If you mean that a jammer doesn'

Not sure what you are saying. If you mean that a jammer isn't tunable, and blasts out energy at all frequencies, you are wrong. If you mean that a jammer doesn't discriminate between devices, I don't think it matters. Guards probably aren't allowed to have cellphones anyways, so it's ok to block all commercial cell phones (if they even exist still). Really, you could block everything except the guards' radios. The prison security system is probably all wired, as well as the computers. This way, the only way prisoners could use a radio would be to communicate with someone standing right outside the prison fence, or with someone in another cell, which is a lot less convenient than the situation right now. If the allowed band is small enough, you could probably just have someone monitor it to catch this.