@ pctechxp

Think Vodafone had a similar issue with the early PAYT SIM cards, I've been told you could get hacked ones back then whose credit would never decrease because the value was written to the card itself rather than it simply giving the network an indication of which account to deduct credit from.

Close but it was Cellnet, specifically the Philips C12 that stored the credit in the handset and I'm convinced (or my memory is..) it was a pic chip inside the phone that re-set the credit when it was rebooted. I assume BT could track them, ie a phone thats made lots of calls but never had credit applied, and killed the sim card in them.