As for totally unknown numbers, I'm not sure if you can, that depends of your phone provider. If he's threatening you or pranking you, you may be able to press some harassing charges. Or even better, just ignore private calls if it's nothing serious.
Sadly, you cant. If you are getting repeated private calls, you may be able to call your service (ex: verizon) and ask to be read the number, but I don't even think they will be able to tell you even if they knew. Sorry.
You can only find out who's calling you in private by contacting your network service provider and giving them details of the time the call was made. They can use a trace line to help you know who that was.