I believe there was an issue with guards needing to be able to use their mobile phones in an emergency. The majority of the Prison service use TETRA radios now which can be linked in to the rest of the UK mainland emergency services so it shouldn't be an issue but there seemed to be a lot of reluctance so that there was a backup in the event everything else failed.
The people I know who work in prisons in the US aren't allowed to take their phones in. At least here the guards wouldn't have phones to use in an emergency.
I think that TETRA are very similar to GSM phones, in both the technology and the frequencies. So it would probably be hard to avoid any jammers interfering with them.
At night the guards occasionally walk around with some sort of signal detector, so something like this already happens. Don't think they can intercept, but they can tell who's using a phone.
It's a tricky one, legally running jammers in the UK is a no-no on the books (http://stakeholders.ofcom.org.uk/enforcement/spectrum-enforc...), however I'm not sure whether there's an exemption clause for prisons, or whether you can apply for a license for a jammer for specific circumstances.
Isn't it possible to separate the trafic? I'm not versed in the GSM standard, but there must be some form of authentication. It might be possible to recognize to whom the signal belongs. (In the worst case, cooperation of the cell service providers could be mandated by law in the network cells covering the prison premises.)
I don't think so, or at least non-trivially anyway. Same reason jammers are prohibited as they have a potential spillover, as you can't definitively say 'this is the range it'll work in'.
On the part of service providers also monitoring it it's difficult, a fair few prisons are actually within reasonably populated areas so it'd have to be pretty accurate signal tracking so you didn't get some spillover.