How do you identify the phone?
So how do the prison authorities identify the phone to the mobile company? If they physically find the phone then they can just confiscate it. If they don't know exactly where the phone is then each prison will need to deploy technology that can triangulate the position of a phone - to ensure it is actually inside - and then pull out its IMEI number. In which case they might just as well deploy jammers or micro-cells.
Sounds like this bit of legislation would be well-worth following up as it is discussed in Parliament - there's a very good chance of some idiotic statements being made for our entertainment.