I once worked in the fraud department of a major US mobile carrier. At that time analog was on its last legs, and the new money-making opportunity was bog-standard identity theft. People would sign up for new phones using stolen information and then rent out minutes to immigrants from the back of a van until we found them out and shut down the phone. We had a system that "fingerprinted" usage patterns such that we knew with a fair degree of certainty when a new account was fraudulent based on his usage patters and the people he called.
I don't see any reason why the same technology wouldn't work to identify criminals with boxes full of prepaid phones.