How old are you? I'm 28, and people my age and younger do this all the time. The younger they are, the less likely there is to be any apology.
But then you also have the people who just scroll through Instagram or something similarly non-linguistic while you're in a conversation. They say they're listening but they "just do this naturally" or "I can do two things at once".
Whether their phone use is blocking or non-blocking of our conversation, it's simply a gesture of disrespect if done more than once or twice. If you don't like my company enough to give full attention, or your other friend is in an emergency, just excuse yourself and leave.
I think I'm going to start excusing myself when I find my friends are doing it.