Even if they answer just to say "I'm in a meeting, I'll call you back" and hang up? That's what I, and most of my colleagues do.
Yep. 'Fraid so. I have colleagues who take that approach and it is still a disrupion to the meeting and they inevitably get asked questions anyway so it's not necessarily a short interruption.
If you can't talk, then it shouldn't be on. The answerphone effectively gives the same message! Same for driving. You can even change the standard voicemail message to say 'i'm driving or in a meeting' etc etc. That way people get an explanation and don't immediately think they are being ignored.
Of course there are always exceptions (serious/urgent calls you are expecting etc) but otherwise IMO it's just part of good meeting etiquette.