Very broad question. There are times when you really don't want to talk to anyone, whether or not in a car or driving it.
I think I chatter away quite a lot whislt driving, but when I need a few seconds extra attention to the road then I go quiet for that period. Perhaps, say, when executing a swift overtaking move.
This is the problem with mobile phones - whether or not hands-free, you caller is not within the environment of the car with you. They cannot have any chance of perceiving what it is you're dealing with at the moment you go quiet.
OK, a lot of passengers stuggle with that as well, but it's much easier to ignore them than it is to ignore some one on the end of the phone who is repeating. "Hello, hello, are you there....." etc.
I often chatter away to myself whilst driving an otherwise empty car. A running commentary to yourself on your driving itself, what you're seeing, how you plan do deal with what's ahead (and behind), in what order, etc etc.
The commentary driving thing is very useful if you want to keep your driving skills polished.
I'd suggest that all torque members try it. But not in front of a passenger you don't know - they will posibbly think you're mad.
In my case, they'd be right. :-(