Try setting your network choice manually. At least on the international version the choose network automatically setting caused me problems like you described. I have not had any problems with calls after I forced the phone to use my operator.
Insanely bizarre phone problem: my Samsung Galaxy S2 suddenly decided to stop ringing or vibrating for incoming calls. I can make calls, and I get/send texts fine, but if you call my phone, it goes to voicemail after ringing once or twice, and my phone doesn't respond. YET, I can look at my call log, and every call is on the list, but with a No Symbol next to it. I thought it might have to do with the "Ditch number" setting for an app called WhoAreYou, but I uninstalled it and this problem still occurs. I've pored through the settings menus looking for any sort of "block calls" or "do not disturb" setting, and I either find them and change them (only to have the issue still persist) or I can't find them. This has been driving me mad, and I'm not looking forward to backing up the videos and pics I have before doing a factory reset. Has anyone else seen or heard or fixed anything like this? Suggestions?