Cell Phone Guidelines

I sometimes commute to North Park University on the train. On the morning ride into the City, the train is quiet and everyone abides by the “keep it quiet” rule. On the ride home in the late afternoon, the guideline changes: You can talk. With one exception:

There are some cars that manage themselves as quiet. The “ssssssssshhhhhhhhh” can be heard if you even try to chat on a cell phone. (I don’t know what would happen if you didn’t “shush.”)

Question: If you could advise high school students and college students and adults how to use cell phones in public , what would be your top two rules/guidelines?

Where is use permissible, where not?