Is it ever impolite to talk on a cellular phone? When?

Sometimes it is definitely impolite. I think a lot of people are very inconsiderate when they talk on their cellular phones, For example, On MRT or bus, maybe someone have take a rest, then you will affect them. In the movie theater, you will affect many people to watch movie. In a meeting, if someone is reporting, when you cell phone is ring, then you will interrupt the meeting. I think that is common sense, but that often has been happened in our life.