OK, bear with me: this one is kind of weird. But I've been thinking about it for, oh, five minutes now, and I've realized that I've never really considered what hand I predominantly hold my smartphone with. I'm left-handed, but my right hand is my dominant smartphone hand. My theory is that this is largely because I use a computer mouse with my right hand, and always have. Not to mention I keep my smartphone in my right-front pocket, so a cross-reach with my left hand would be really awkward (not that I couldn't easily keep it in the left pocket, though).
However, there is also the question of phone calls: I am absolutely religious about holding my phone up to my left ear on a voice call, obviously holding it with my left hand. I don't know why. Holding it up to my right ear seems outright heretical, and feels deeply uncomfortable whenever I do it. And so, I think there may be a strong hand preference related to phone calls (granted, most of us make far fewer of them these days).
I've also noticed a great many people may use one hand to cradle their phone, while using their preferred hand to actually interact with the screen. Does the traditional left/right dominance of the cradle hand vary? And, as compared to the predominance of left-handed of a whole in the human population, is right-handed dominance more or less prevalent in smartphone use? (The generally-accepted estimate of left-handedness is around 10% of the human population.) I realize our results here are far from scientific (not to mention, heavily skewed demographically to males ages 18-40), but I am curious.