If the pain is in your shoulder then the phone is below the points where the nerves travel to the shoulder and annot be pressing on them.
That doesn't mean it isn't responsible for the pain, it could be doing it by another mechanism eg squishing the muscles.
Pain from a "trapped nerve" is far more likely to be coming from the neck but shoulder pain usually has another cause such as tendinitis, capsulitis, arthritis. The treatment of these is usually Physio and simple analgesia.
I have recently had awful shoulder pain and it only got better when I made alterations at work and did lots of Physio. But t still comes back if I spend too much time on my iPad...
