There is a fix for stupid and rude: Education.
There's also a technical reason for people talking louder on cell phones than they do on land-line phones. People gauge their speaking volume by what they hear. The designers of the land-line phone system figured this out, and phones provide feedback to your ear (what you say is played back to you instantly in the earpiece - the ear that you're really paying attention to) so that you automatically (without knowing it) know how loud to talk. Cell phones don't do this, which results in people speaking more loudly than they need to without realizing they're doing it.
Back to the education: Knowing this, it's fairly straight-forward for someone to monitor their volume using their other ear and keep themselves from getting too loud (at least until they get really excited about something and forget).
