People who keep their phone face-down on the table often aren’t being polite, many learned early that being reachable was the fastest way to keep the people around them calm, and the gesture is the only boundary they can enforce without having to explain it
The face-down phone looks like good manners, but for people raised to feel responsible for everyone else's needs, it's something quieter and more important: a small, unspoken refusal to be on call.