What you should do is duplicate your 3d stroke layer (make sure its not in 3d mode)
Put one layer in front of your phone, and one behind.
Play with the 3d stroke camera Z clipping settings (ie. leave Z clip front at zero and set the Z clip back to a smaller number than the default (try 400 for example))
Note what it does to your stroke…
Once your find a setting that clips your stroke to look like only the parts that would be in front of your phone, take that value and put it into the Z clip front setting of your back layer.
The key is to understand how the Z clip settings effect 3d stroke.
Hopefully that makes sense…