Phone Voices

I don't understand why some people do this, but it appears that some believe it's necessary to speak differently on the telephone. Let me explain. Some people (case in point, my mom), use a completely different tone, intonation, and expression of emotion on the telephone. I have no idea why someone would be compelled to do such a thing. If you're in a shitty mood and have sounded monotone all day, why do you want to sound all giggly and happy to someone on the phone? Does it matter? I'd be interested in hearing theories about this phenomena and if anybody else has encountered this before (or actually does this).