The screen is a mirror. But physics don't work in a dream other than what you observed and can replicate manually. Every experience of physics within the dream are generated manually, piece by piece.
Some might say you don't walk or run, you float or float faster. Others would say you don't even do that, but rather the dream world moves around you.

So when you look at the screen of a phone, it's like a dream within a dream. It has to be crafted and you get sent within the screen, turning that into the actual dream.

If you've ever fired a gun in a dream, you'll note that it usually doesn't work. Or if it does, the targets drop without you "feeling" the entire process start to finish. And if you focus on the strangeness of this, then next time you fire a bullet, the knowledge you have of how that works gets added. So you see the bullet coming out of the gun, you follow along as it travels and you see it hit the target as you've seen it before in real life, whether it's actual practice, a game or a movie.
However, everything else comes to a standstill. The you that shot the bullet is left behind or no longer there, the target being shot waits there quietly to receive the bullet or even actively cooperates and the background world no longer matters.

The dream world isn't fixed, tightly bound by the rules of the real world. The dream world is what you focus on at that moment and everything else changes to fit. So if you don't see a cellphone in your dreams, it's because they're not relevant to you persistently as a cellphone and they instead become (the gateway of) what they show you as soon as you focus on that.

I have tech of all kinds in my dreams, including smartphones, and that's clearly not rare since there are always many comments saying so every time this gets posted.