> You can, if course, just measure and draw it out yourself. But the rounded sides are likely going to be guesses.
Use a caliper measuring device and a flatbed scanner.
With the flatbed scanner, make a high res (600 dpi, or maybe even 1200 dpi) 2D scan of the front of the phone. Keep the screen of the phone on while scanning, and have it display some image that contrasts well against the edges of the screen, so you can clearly see where the edges are. (Ie if the phone is black or other dark color, use a an all white image. If the phone is a bright color, use a dark image.)
Scan the back of the phone as well, so you can see where the camera lenses on the back of the phone are.
Import the 2D images into your 3D modeling software of choice and create a flat polygon covering the front view. Extrude into a 3D shape.
Scale according to physical width, height and depth of the phone that you measure with the caliper.
Make room for the lenses using the scan of the back of the phone.
Buttons on the side and charging port on the bottom you can decide on how to fit from using caliper measurements, and adding some additional room around those as that is often desired.
Then model a shell that fits around it.
It will probably take a couple of test prints and refinements to get it right even so, but I think said approach will work well in terms of benefit to effort.
If you’ve got curved edges that matter (my iPhone13 doesn’t, but my PinePhone does), it might be worth trying to get a good end-on cross section photo, and importing that into a 2D drawing tool you can use to create that shape in a format your 3D modelling tool can extrude. Looking at my PinePhone, and thinking in OpenSCAD terms, I’d probably model the phone as an intersection of two extruded objects one from the end and one from the side, where the 2D shapes I’m extruding would be as close as I could get to end and side profiles, then use rotate-extrude to make the rounded corners (these look “circular enough” for that, if they were more elliptical, it resize the circular extrusions to suit).
That’d work pretty well for phones (like the PinePhone) that are basically “rectangular prism slabs with uniformly rounded edges and corners”. It’d be much harder to model this Samsung S4 I have here, with its curved ends and it’s compound curved back…
Use a caliper measuring device and a flatbed scanner.
With the flatbed scanner, make a high res (600 dpi, or maybe even 1200 dpi) 2D scan of the front of the phone. Keep the screen of the phone on while scanning, and have it display some image that contrasts well against the edges of the screen, so you can clearly see where the edges are. (Ie if the phone is black or other dark color, use a an all white image. If the phone is a bright color, use a dark image.)
Scan the back of the phone as well, so you can see where the camera lenses on the back of the phone are.
Import the 2D images into your 3D modeling software of choice and create a flat polygon covering the front view. Extrude into a 3D shape.
Scale according to physical width, height and depth of the phone that you measure with the caliper.
Make room for the lenses using the scan of the back of the phone.
Buttons on the side and charging port on the bottom you can decide on how to fit from using caliper measurements, and adding some additional room around those as that is often desired.
Then model a shell that fits around it.
It will probably take a couple of test prints and refinements to get it right even so, but I think said approach will work well in terms of benefit to effort.