Experientially, I concur with Texrat. To me, it 'feels' like I'm engaging more of my brain in concentration when I'm talking on the phone while driving, rather than talking to a passenger.

However, this difference 'feels' less when I'm using a bluetooth headset....and even less when the phone is tethered via bluetooth to the car audio system (it feels just like an in-car conversation).

I think part of the difference is due to the crappy audio quality coming from the phone or headset....your brain is concentrating on interpreting the sound....whereas with the call coming through the audio system, it sounds clear and easily intelligible.