iPhone 4S GSM full bar with 3G but not working

Hello everyone.

Here is an issue I can't beat.

iPhone 4S, not locked to carrier. Suddenly I noticed that may people start to complain to me about my phone is switch off. But same time phone is on and has full GSM bar with 3G. Once I make an experiment, when my friend Skype me about my phone is off i tried to make a call, but call is won't connect, i even can't hear a beeps. I tried to reboot the phone and in a few hours story repeats. Then I tried to resent network settings. Won't help. I tryed to restore my iPhone with iTunes with latest firmware. Not luck. Last thing I did is replaced my SIM card with my carrier. No luck. I saw on internet, that there are a lot of people having same issue with no luck of fixing it. My phone is on warranty, but or local dialer won't replace it, because it's bug not consistently seen. Any suggestions?