The first point I'd like to address is when not to use your phone. It's polite to switch off you phone or turn off the sound when you're in class or in a meeting. If you get an important call, you should ask for permission to leave the room and then don't talk for too long. Furthermore, for conversation that need more time, it is best to ask the person to call back at a more convenient time. Cell phones can also cause you to neglect the people you are with. I find it really annoying when my friends constantly check their messages on their phone. In fact I want to tell them to turn off the cell phone and enjoy my company!