Do most, if not all people, have a "phone voice"?

In my experience, pretty much everyone I have ever heard answer a phone, whether for business or personal reasons, has done so with a voice different than their typical voice. I know I do it. When I was a kid, it used to irritate me when my mother would be yelling or bitching at me and then pick up the phone and have such a pleasant voice.

Is this your experience, as well? Why do you think this occurs?