@Fredd...Cheers
Sketchup is powerful, but many people don't have the time (or take the time) to learn it.
@val5214 Let me say this first...complex curved faces in Sketchup are
hard. Sketchup was never intended for designing them. That said you should be able to get reasonably close.
A set of photos to trace is likely the easiest way, if you can get good ones. Try to take to photos from some distance away so there is less lens distortion from the camera you're using to take the pictures.
Another thing that may work well is using some kind of modeling clay (
Crayola model magic works well and isn't sticky) to create an impression, then measuring from that. If you do this, you'll want to mold the clay into a sort of cross section so that you can measure it. So rather than putting clay all around the phone, just put it on half of the back so that you can measure it with your calipers. Does that make sense?
You may also use this awesome Sketchup plugin:
Curviloft. You'll need to sign up for a free account to download it. If using this one, create a set of curves based on 'slices' of the phone, then use Curviloft to join them. If you use the modeling clay, you could make a single mold of the phone back and cut it into slices to recreate is Sketchup.