There is a project to implement technical cell phone control in prisons - but it is not simple or trivial.
The cost to faraday shield prisons would be huge and probably pointless - there are easy solutions to bypass physical shielding.
Electronic methods are just as hard. Imagine the media outcry if members of the public outside the prison boundaries discovered that their calls had been monitored by prison officers or simply jammed due to security concerns. It is very hard to manage controlled RF coverage within a specific area - if environmental conditions change, then the covered zone could grow or shrink by large amounts. The same applies to RF triangulation - there are too many reflective surfaces and distorting materials to accurately localise a transmitting device quickly, and a simple external aerial on a wire would provide a prisoner enough time to disappear a phone somewhere secretive.
However, there needs to be a legal framework for such communications control as well as the technical solution. Currently no jamming is legal - there needs to be some legal method for this to occur, even for the government.
So the fastest approach to reduce the supply of these small phones are via the routes suggested - electrical safety and trademark law. It is intended to slow the supply of these devices _until_ technical measures can be taken to prevent the use of these phones in prisons.
There is no requirement for staff or visitors in a prison to use mobile phones - goes with the territory.