Holding the phone while you use it is not the problem, according to a study by psychprof David Strayer. This doesn't surprise me. Talking on the phone takes the visual part of your brain into the world of imagination.
Just the other day, I was talking to one of my colleagues about this as we were watching Bloggingheads -- which, you know, I was on. You're hearing the other person over a phone line and not seeing him, but you want the expression on your face to look like you're seeing him. It's somewhat hard to do, because the natural tendency is to have "phone face" (a term I'm making up). You're eyes are open but they've gone rather blind, because the brain connection to the eyes is not there. The brain's visual center is working in the imagination mode, summoning up images of things that aren't coming in through the eyes. That's how I'm explaining it to myself at least.
I think when you talk on the phone, you are generating a temporary blindness. It affects the way you look on Bloggingheads, and you need to do a little acting to disguise it. Of course, to do that is to use even more imagination, not to turn your eyes back on. When you're driving, we mostly don't care how you look, but you really do need to see. How are you going to keep your brain from disconnecting with your eyes?
Speaking of Bloggingheads, by the way, Bob Wright and Mickey Kaus talk about me in the beginning of the new episode (which also has other good stuff in it). The part about me starts at 2:23. Referring to my Bloggingheads performance, Bob says, "She was very civil. I mean, maybe she has a latent combative side that could be brought out by a sufficiently provocative interlocutor." He says he's "hoping to provoke an ill-advised outburst" from me on the show, and Mickey's response to that is "But she's blonde." Is he trying to provoke me?