The proximity sensor being too sensitive and turning on when you have the phone to your ear, which can do things like hang up the call, turn on loudspeaker, etc.
I read about this problem on these forums I think, someone also posted a link to another forum where this issue was bought up but there was a relatively simple fix for it. Can't remember for the life of me where I read it though.
I have since changed from o2 to 3 and its the best, I can't believe how good it is. Get h or 3g everwhere, really impressed. if anyone is in a one month rolling contract I highly recommend trying out a one month 3 sim, hell just order one and then send it back within 14days if you wanted, nothing to lose but potentially a lot of h to gain 