That’s a good observation. I think it’s where their heads are facing. Two conversationalists face each other or face the room, but talk quietly with body language that suggests they’re conversing (heads tilted, leaning.)

Someone talking on the phone but facing away from the room isn’t usually considered rude, either.

In a waiting room or train car, if you’re sitting and talking on the phone or Zoom, you’re broadcasting into the room with your posture. Your voice tends to broadcast as well with no wall or other conversant as an immediate barrier.

I would consider two people talking to each other in a waiting room or on a train very rude too. If they are loud enough that the people next to them or behind them can hear the words they are saying, then it is rude of them. Of course, this is relative to how noisy the place is overall.