although everything you know says its not possible for plastic to stop a bullet there is a metal chip inside.
Also anything in the way of the bullet flying through the air can stop the bullet or at least slow it down. The phone is made with a dense material which once hit can cause a bounce, block or redirection of the bullet.
Although the phone is an unlikely match for the bullet. it is possible that it could stop the dirction of travel which i believe is what happened here.