So it's not a matter of talking being distracting at all then, anyway? It's a matter of knowing when to return 100% of your attention to driving. That sounds like it's based on the judgment of the driver, NOT the cellphone. I can, and do, tell the person on the cellphone to "hang on" or "I gotta go - *click*" if and when I need to focus 100% on driving. Just like I tell the people in the car with me to STFU when I need to do the same.
It has nothing to do with the cellphone.