[punky]

Maybe a better way of doing this would be to make the prison a Faraday cage.

That way you don't have to disrupt phone signals for civvies but it means no signals will go in and out of the prison. However it means radios and such will work inside within the prison. How often does a guard need a radio to contact outside the prison? If there are sentrys then they can be hard wired.

There is a cost to this but it means the prisoners can't switch to radios with some guy in the back of a van near the prison receiving the transmissions.