You can do Device Emulation in Roblox Studio, as it has things like iPads, other tablets, and VGA (square screen computers). If you didn’t know about these, then I recommend giving it a try and looking at the position of the phone GUI to see what’s wrong with it and why people on these devices can’t see it at all when it’s in its hidden position.
That video is interesting, but I don’t actually know what’s causing that, as I don’t know what the phone GUI’s properties are set to, so I’ll be making assumptions.
My assumption is that you have the phone GUI all inside one Frame. I’m also assuming that the Anchor Point is set to (1,1) and the position set to something similar to {(1,-15),(1,BigNegativeNumber)} when the phone GUI is in it’s hidden position. I can see that the phone shrinks as it slides offscreen, which is what made me guess you are using a negative Y offset position for the hidden phone’s position and that the anchor point was somewhere near the bottom of the phone.
If it is true that the phone is all in a single Frame, then you could change the Frame’s Anchor Position to be (1,0) and use the Y Position Scale to set it a certain distance (like 0.9) from the top of the screen and set the Y Position Offset to 0.
If I got this wrong, I’d recommend taking pictures of the properties and maybe even taking pictures of the phone GUI’s descendants in the Explorer. I think it might help someone identify what the issue is.