Re: how about..
"The solution is to use femcell"
I've always wondered why they didn't just blanket jail and prisons in a fine mesh, then implement a femto-cell within the cage. Set it up so that during the connection process it brings up a message stating that the local network is being actively monitored. Maybe just inform the person through a text message that the phone owner would answer:
"I consent, but check with me later",
"I consent and don't ask again"
"I don't consent, but ask again later", and
"I do not consent, never try to allow my phone to associate again"
With it being in a Faraday cage, people who do not consent to the monitoring aren't tracked and their phone just doesn't work while being able to track the phones of people who do consent. For people being held, they'd either have to consent to being tracked or to go without connectivity.