Hi ive had exactly the same problem as you and when i took my phone back to 02 they said it was water damaged, i have since sent it off for a repair with another private company and they have said they are 95% sure its a software fault due to the way my screen was turning black and then saying insert sim, but i shall let you know when i get it back if it has fixed it or it could be something like the sim card reader but please dont be fobbed off saying its water damage they shall just drop a bit off water on the water tab and say its beyond economical repair