Add a Sprite3D node as a child of your phone mesh and position. For images, all you have to do is drag the image into the texture on the editor and you're done.
To play a vid, add a Viewport as a child of the sprite and then a VideoPlayer as a child of that. Add a new viewport texture in the inspector of your Sprite3D. On the viewport set the render target to v flip. You might have to hit keep 3d linear, if you get shading artefacts do that. Then set the resolution to match whatever your video is. That's the size value.
Touch wood that should work. Any issues let me know.