How to say “On the phone” （手机里 vs. 手机上）
手机上 describes the act is observed by something from outside of the cellphone through the screen. I guess most of the humanity will do this way.

手机里, in contrast, suggests something is really inside the phone, e.g. in memory, storage etc.

So, 手机里的照片 "pictures (stored) on the phone" and 手机上的照片 "pictures (found) on the phone" may indicate the same thing. But if you'd say 我在手机里玩游戏, I must assume you're a relative of Siri.