From the article: The police, however, are slightly different in that they can go back to the network operator later and link the TIMSI to a real IMSI. That will generally link to a physical person, who might then have to explain what his/her phone was doing at the time in question.
That will work extremely well if all the illegal phones in the prison are conveniently on contract or are registered PAYG. Somehow that seems rather unlikely.
Given that many of the illegal phones may have been previously stolen from their rightful owners it becomes extremely important that any theft or loss is reported immediately so that any later illegal use can be shown to have taken place after the theft or loss. There is also a case that any phone and / or SIM that has been replaced should be destroyed so that it cannot go through a second hand chain and finish up being used for illegal purposes; that could all to easily come back to haunt the original totally innocent owner, and while proof of guilt is for the prosecution and the courts to establish just saying "it was lost / stolen / sold on" without being able to prove that statement could become a source of undeserved pain.