I really dont see where you're coming from. To see body language you need to be looking in the general direction of tha passenger - and when driving thats certainly not where I'm looking. Maybe its just me but I can bearly even see anything but the legs of my passenger while im driving. If i want to see them i have to look left, which means taking my eyes off the road. I dont have to do that and i dont feel obliged to look the person speaking when they are a phone! Also why do you have to concentrate more to pick out whats being said? Do you have particularly bad cell coverage there? Its a clear as a whistle and right in my earphone when im using mine - i might look like a star-trek reject but thats a side point! All i have to do is sit back and chat as if i was talking to myself?? With multiple passengers you feel the need to look in the mirror to talk to them - but since I cant fit anyone in the back of my car i dont really have that either!
I suppose its a personal thing but IMO if there is someone there to talk to you feel the need to look at them, its human nature and you just pointed it out. If theres no-one there and you have good coverage so the line doesnt break up (99% of the UK) a phone takes less effort to talk to and requires no visual contact.
J