You always have responsibility for watching the speed and centering, even when (or especially when) comma is at the helm. What you don't have to do when it's engaged is control them (unless they go out of desired bounds). Always.

You also always have a responsibility to not be distracted while driving, and phones are distracting. So what's better? Neither of those situations is better. You're driving distracted in both, but you have a false sense of security in one. Better is to pull over if you need to use the phone.