...from way back!
My personal rule has always been, "Do not interrupt the person you are talking with for someone else." Unless. of course, it's the boss, or my kid (or her mom) with an emergency. After that, flames, not smoke, and only for arterial blood.
I watched a fair number of folks get real uncomfortable when I ignored my phone and continued our conversation. I never actually timed anyone, but my best guess is that it was less than a minute before I was asked "Aren't you going to get that?"
I suspect that anxiety is behind the compulsion to be in constant contact (and not the other way around, as stated above). So I suspect that some people are just not comfortable in their own skin.