I really don't see much difference between making calls and other tasks
e.g. in my Skoda Octavia - the functionality to make a call on the screen in the dashboard is as similar, if a little fewer steps, than changing from e.g. radio to a track in a folder stored on the SD card for music - I can see no difference in the concentration / distraction / etc. in doing that...
once into the call (which really is very good quality) it is probable that the level of distraction is less than for example having to resolve squabbles from children in the back seat which I think is far more distracting... I accept that it is probably a bit more than a passenger as you tend to focus more when you can't see the person you are talking to... but I am sure there are plenty of drivers and passengers who have very intense discussions!
even operating a physical mobile phone (which I wouldn't do) is probably less distracting than lighting a match / cigarette lighter, popping a cigarette into your mouth, holding a naked flame to your face and lighting the cigarette - all while doing 70+ up the motorway
It has to be less of an issue than the lady I once followed on the M5 who was (with considerable talent!) changing her baby's nappy on the passenger seat (in it's seat) while driving... I hate to think what contortions were required, but it showed a very patient baby and a physically talented mother (albeit mentally a little suspect! and she did manage to occupy all three lanes of the motorway while doing it!) The point being there are lots of stupid and distracting things people do - mobile phones can be bad, but there are also ways of managing them (e.g. through the car system) to minimise issues...
surely the issue has to be any loss of concentration / distraction - not the actual action per se?
Alasdair