He didn't mention if there was a lack of RS or PC or not. Maybe he was speeding.
But the answer is, there is no definitive case precedent for the answer nationally or even in a particular federal circuit (maybe your state has a state court decision.) My professional opinion is the reasonable person standard applies. The law gives a lot of leeway to an officer to control a situation for reasonable concerns during a lawful stop and I don't see this as any different. If the cell phone use delays or obstructs the officer's duties or there is articulable concern that the subject is calling someone to come to illegally interfere with the lawful stop then you can restrict their use and demand compliance. Otherwise no.
So if I am trying to talk to you about whatever I have stopped you for and you have the window rolled up while you call your attorney/mother/bail bondsman/whatever, you are going to experience additional unpleasantness. If I tell you to keep your hands out of your purse in an area or situation where I can articulate there is danger of weapons being used against me, and you insist on grabbing your phone, that isn't appropriate. If you are a known gang member and I have articulable concerns that you might call your bros to come to the scene and cause trouble I might even be justified in seizing your phone for a period of time.
But if you want to call the judge from your car (or your wife or your voicemail) while I'm back writing the ticket, knock yourself out. Hang up when I come back to your window or expect my lawful demand that you do so.
That's my guidance until the SCOTUS or the Ninth Circuit (in my case) says otherwise.
Everything you need to know in life you learned in Kindergarten-or in my case first grade.
