The phone in company, and what a table can read.
Placed face up, face down or out of sight, the object is saying something either way.
A phone on a table is never neutral. Face up, it announces that the conversation may be interrupted and that the interruption will be permitted, and everyone at the table understands this without discussing it. Face down softens the claim without withdrawing it. In a bag or a pocket, the object says the next while belongs to whoever is present. None of these is a rule of etiquette handed down from anywhere. They are conventions that assembled themselves quickly, the way conventions about hats or handshakes once did, and people read them accurately even when they would struggle to describe them.
The awkwardness usually comes from mismatch rather than from the device. A person checking something practical, a train time or an address relevant to the meal, is doing something entirely reasonable, but from across the table the gesture looks identical to drifting away. The remedy that everyone eventually discovers is narration: saying what you are doing, briefly, out loud. It costs three words and it converts a private absence into a shared errand, which is the whole of the difficulty resolved.
There is a related courtesy that has less to do with attention and more with the room. A speaker on a train carriage, a call taken in a quiet cafe, a video played aloud in a waiting area: these are not attention questions at all, they are noise questions, and they are old. The rules for them predate the object by a century. Anyone who knows how to behave in a library already knows how to behave with a phone in public, and mostly does, most of the time.