What I've noticed with my university/adult students is that looking at phones has now become almost a tic - they don't know they're doing it.
I have a pretty low tolerance for cell phones in class, and all my students know this (in my adult classes, however, I do know that occasionally moms will need to check their phones, or other exceptions need to be made. I'm not merciless). I was teaching one day, looking directly at a student, when she broke eye contact with me, leaned over, picked up her phone, checked the screen, and put it back. The following is our (paraphrased) conversation:
Me: Alexandra! What are you doing with your phone?
Student: (Looking surprised and embarrassed) I'm so sorry! I don't know why I did that!
Me: Why you looked at your phone in class?
Student: I didn't mean to! I think it's just a habit.
And THAT'S the crux of the matter. People are constantly on their phones out of habit, I think, not realizing they're being rude, because they're just not thinking about it.
Up until June of this year, I had a regular old phone I could use for calls and texts. In June, I got an iPhone. While I love it and, purely for the Livestrong app would never give it back, I have to constantly remind myself that I don't have to constantly be using it, just because I have it. I try NOT to use it to fill time when I'm waiting for a friend, or in line at the store. I think it just encourages my short attention span.
Whew!

All that to say, I really try not to use my phone when I'm with other people, and would be ticked off if they stopped having a conversation with me just to text someone else.