I smile compulsively when I'm on the phone at work. That's how I make myself sound pleasant. I think. At any rate, I do sound pleasant (unless I’m almightily miffed), and I do smile. It's worse on the short phone calls, probably because a short phone call almost always has a much higher ratio of pleasantry to substance than a long one. Sometimes I hang up the phone and my face aches slightly from the smiling.
I'm not sure if I need to do something about this or not.