Nokia Nseries :: N8 - Sometimes Calling Doesn't Work

i am unable to make calls. When I try to make phone call it doesn't even try to make the call. I choose a name from the address book to call and nothing happens. After restaring the phone it works fine. An even more annoying problem is that when this has happened my phone doesn't receive calls either. I've heard that people have tried to call me and it has rang normally but my phone doesn't show that someone has called me. Otherwise the phone works fine so I newer know that it is in this "problem state" until I try to make a call.Software version is 025.007.240.02