did you check the mast status near you? http://status.o2.co.uk/
Is it possible for you to try your sim in another phone? Or try another sim in your phone? This way we can narrow down which is causing the problem.
Have you made any recent changes to the phone that may have caused the problem?
What happens when someone tries to call you? and is this a problem with all numbers or just one person?
If you get back to us it will help greatly in diagnosing the problem