That's interesting, thank you. I wonder if there's similar research on talking to passengers?
I read a study a few years back where it was shown to be much more distracting being on hands free than talking to passengers for a few reasons.
A passenger can see if there's a junction / hazard up ahead and is likely to stop talking as a result. If you pause on the phone when making a quick decision around lane changing etc. you might get a "hello, hello, are you there, hello?" so there's a lot more demand on your concentration than with a passenger.
I was on the phone to a colleague and he managed to take the wrong lane out of bristol and headed to london instead of manchester - only an annoyance for him in that case, but he wouldn't have made the same mistake if he hadn't been on the phone.
Our workplace, and a lot of others now explicitly ban mobile phone use in the car excepting for navigation or to keep emergency alerts on in the event of a crash.