iPhone 4 shows No Service but can use internet

Hi, my iPhone 4 begins to show 'No Service', but internet and wifi is still working fine.

Does anyone experience such problem?

I already checked on O2, and after they tried all ways including new sim card, and found out it's not the problem of sim. So should be the phone problem I guess.

So generally, I can't use messege and call. Does anyone know such problem? Thanks.