Is it normal to hear the other side of a cell phone conversation?

I've noticed that a lot of times when someone is talking on their cell phone, I can hear the person on the other end talking. It's always very loud, even louder than a normal tone of voice. Why is this so loud? Do other people experience this? This doesn't happen when someone is speaking on a regular phone.
This is to people and their loud cell phone conversations: I don't want to hear your conversation. I am sooo not interested. Take it outside.