iPhone 4S keeps searching for carrier

Since yesterday my iPhone 4s (iOS 5.0.1) keeps searching for carrier. When it finds one (ussualy after a few minutes), it starts searching again for carriers after a few minutes. It sometimes even displays "No service".

It's not due the sim card. I've tried the sim card on a different device and it works. I've tried to reset the iPhone by restoring it, does not help. Toggling the airplane mode does not help.

When restoring the phone I received the message "This iPhone may be damaged and cannot be activated for service Please visit your nearest Apple Store for more information" during the activation wizard. I did the wizard over again and passed it. I did not experience any problems since the purchase months ago.

I did not drop the phone recently or anything else that create this issue. Has anyone experienced this kind of issue?