From what I've experienced and what I've seen it definitely seems to be related to the 3G – have you tried turning that off in the Settings? I've had countless missed calls and failed calls while looking at 5 bars and not touching the iPhone at all. Turned off 3G and suddenly things seem to be working. I have to agree with Innofish that it must be a software issue and I wonder if that's what Apple's press conference tomorrow is about.