Is it still me that's of the opinion that if you need to track mobile phones in a prison, that the prisoners shouldn't be able to get hold of them anyway?
Fixing entirely the wrong problem.
Sure, this might catch a few (stupid) over-the-wall-throwers, jailbreaker assisters, etc. but they will smarten up quite quick and just leave their phones at home that day.
If prisoners having phones is a problem, stop them getting phones.
And if a dragnet like this is anything other than "there's a new phone on the site!" "Well, where did they get that from, trace the history and see if we can work it out", then you're really on the wrong side of the problem.
If you can smuggle a phone, you can smuggle a knife or even a small bomb.