A couple of friends of mine have had this happen on their prepay phones during the day, and then the phone being unable to work till power cycled. Happened to me a couple of months ago but I thought nothing of it since I was just using an old sim which had no credit left on it I though it might have being vodafones way of telling me to top up.

Anyway, the people it has happened to also have no/minimal credit on the phones in question, so is this a new phenomina that vodafone are doing or is it just a werid coincidence?