My car is equipped with a Bluetooth device that interrupts the sound system when the phone is active. Talking on the phone for me is no different that talking with someone in the passenger seat.
My TomTom is like a backseat driver saying, "After 400 yards, turn right", etc... Even if a so called "smart phone" can manage the GPS functions while I am talking on the phone, the audio GPS directions would cause cut-outs in the conversation. That is not desirable.
I recently drove from Portland OR to SF BayArea and was re-routed around traffic backups while in transit. This was with the TomTom Live system. Will phone based GPS apps do that and let me talk on the phone? I don't get this rush to put everything in a phone.
The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth. -- Niels Bohr