I can't stand when people yell into their cell phones. But I probably do a bit of yelling myself when I'm on my cell phone. You may already know this, but the technological explanation for this is in the existance of a sidetone.
A sidetone plays your own voice back into your ear, creating the illusion that you're talking louder than you actually are. House phones have this feature, but cell phones do not. Why? Good question. When cell phone manufacturers begin implementing sidetones into their new hardware, I will be happy. And the world will be a more peaceful place.
I learned about this for the first time in a very good NPR story. You can listen to that same story here. It's less than five minutes long.