Apparently phones are being smuggled into prisons at an alarming rate, and it seems like a losing battle trying to keep them out. So if you can't keep phones out of prisons, you can make them useless with a faraday cage, which prevents radio waves getting in or out. And this is exactly one of those circumstances where such expensive measures may be justified. Rather than spending a fortune trying to search for phones in prisons, admit limited defeat on that front and prevent their actual use. Or set up a local, fake phone tower for them to latch onto, which just swallows up everything that goes into it. That might be a problem for anyone living, working or driving by the local area, though.
Mokalus of Borg
PS - Both of these measures would adversely affect guards and visitors, too.
PPS - Maybe that's acceptable.