lucy24 - 3:51 pm on Jul 8, 2013 (gmt 0)
It's a pattern learned in face-to-face conversation. Unless you know for a fact that the person you're talking to is hard of hearing, you have to assume that the two of you can hear-- or not hear-- each other equally well. I've caught myself dropping to a whisper when speaking to someone with laryngitis. So far, mercifully you don't see a lot of toddlers with cell phones. That's when you learn conversational patterns.
The raised voice is also more noticeable because bystanders only hear one side of the conversation, so inevitably it goes UP and down and UP AGAIN and down again... which may be a ### of a lot more annoying and distracting than standing next to someone shouting continuously. Every time you think they're done they start up again.
Rhetorical question: Have you ever found yourself in the grocery line behind someone having a phone conversation about something urgent and important?