No signal bar but carrier shows and 3G internett works

Hi,

I've got an iPhone 4 16 GB (no jailbreak or locked to carrier). When I but my sim card in my carrier shows on the top, but no bars/signal. I am abble to surf with high speed thru 3G, but not abble to call or text. I have tryed to restore in DFU but it does not work. Please help me!