Who cares....
"Her defense team will argue she was checking Slack messages from Uber in her work phone at the time, whilst prosecutors will say she was watching an episode of reality show The Voice on her personal handset."
It doesn't really matter which, it matters whether either of these was considered acceptable whilst being the legally responsible driver in a vehicle.
If checking slack is something which was accepted, or even expected, by her employers then that is a serious issue.
She was a safety driver, not a systems engineers meant to be heads down doing other work whilst the vehicle was in motion.