Can a professor tell you that you can’t use your phone before or after their class?
I think your description of what’s going on is misleading. The professor is not “telling you you can’t use your phone before class”. He’s telling you you can’t use your phone before class in the classroom when he’s also in the classroom.

Is that reasonable? I honestly don’t have a strong opinion. I can certainly see why it’s annoying to you on the one hand, but can also see things from the professor’s perspective - if he wants to use those thirty minutes before class to review the material he’ll be discussing, and is easily distracted by phones (yes, professors are prone to distraction just like other people), he may well think his need to have a distraction-free working environment so that he can deliver good instruction for his class overrides your personal need to use your phone during that time and makes it okay for him to decide what is and isn’t allowed in the classroom, even though the official lecture time hasn’t yet arrived.

It’s possible you can win this on a technicality by turning this into some kind of principled fight over individual rights. But I don’t see a point, it wouldn’t be a real “win” for you or anyone else in any meaningful sense. Your professor may be a slightly unreasonable person with a slightly rigid personality, but in my opinion the mature thing to do in this situation would be to respect his wishes and find some other place to use your phone before class.