Yes it's the content delivered by the phone. My first mobile phones could only make calls. Not even text messaging was supported. So pretty obviously you're not going to pull out your phone and call someone during a lag in conversation unless you are just supremely rude or maybe it's a call to invite someone to come over and join the group. You might answer a call if you get one I suppose, but it would be fairly awkward. I do remember the people who always seemed to be on a mobile call, often with a headset of some sort, and thinking they were complete douchebags, but they stood out by being few in number.
As text, email, other messages, websites, Facebook, etc. became available the draw became stronger and so did the addiction and the normalization of looking at your phone every 30 seconds while you were with someone.
Did SNL or anyone ever do a skit of a couple having sex and then "ding" a phone chimes and one of them picks it up and starts reading the message? And then the other one grabs their phone and starts scrolling?