This happened to a colleague of mine, but I'm on the same network and it didn't happen to me. You can turn it off by either going into an O2 store with some ID

This happened to a colleague of mine, but I'm on the same network and it didn't happen to me. You can turn it off by either going into an O2 store with some ID, or paying £1 on a credit card - you'll be given £2.50 in credit to your phone bill to compensate.