Why people speak louder on the phone when they can't hear the opposite person?

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Think about this.. My mom & many other people I know tend to speak and yell on the phone when they can't hear the the person on the other end speak... I mean hw would you speaking louder make him louder? :-S You don't want to hear your own voice louder.. do u ? ;-) hehe.. i've always found this human behavior weird..