Re: how about..
"Putting a fine mesh around the cells is all well and good, but what about when the cell door is open?"
He didn't say per cell, he was talking about the cage (a Faraday Cage, see: https://en.wikipedia.org/wiki/Faraday_cage) being the entire prison. The outermost walls having the actual Faraday Cage properties and components. Hoisting a mast in the prison yard, or trying to stick an antenna outside of the outer wall will be trivial to spot and manage, AND random citizens will not have to have their radio transmissions searched and seized by the titheads. Go ahead and quietly accept this, and soon you'll have every government agency and their dogs monitoring you and deciding whether or not you are doing something they don't like, wherever you live, not just near prisons like the "test population" of people too poor to legally fight back.
I have the NSA/CIA/DHS monitoring all my traffic "for my safety" and the FBI flying overhead with Stingray cellular spy "towers." In both cases they are not "cleaning the streets of terrorists," merely infringing further on the few rights that the citizens still cling to, and that they (FBI, et al) secretly oppose. This is NOT what the founding fathers had in mind. This is spying under the guise of the latest boogeyman (Russians, Muslims, etc) but the end result is my privacy is circumvented.
What would happen if someone could engineer a super low-cost 2/3G fake celluar device that clogs up the works of these fake towers? Real tower? Pass. Fake tower? Keep asking for authorizations to DoS it, then flood the airwaves with fake traffic, or inspect the other devices, etc. Sure, it's illegal to setup a cell tower... I'm taking about fake devices, and hundreds of them for ~US$10 each.
I'm looking at an online catalog and they have some GSM/GPRS and Arduino "Cellular Shields", but those are US$35 and US$70. There's got to be a cheaper way, like reusing old castaway phone, etc. Hack 'em, if they can't take a joke!