Cell phone noise

Why do cell phone conversations stick out?

I'm sitting in the library and someone across the room is on her cell. It's extremely distracting. She's breaking no rules; this isn't a quiet study room, and other people are talking, but only the person on the phone is distracting.

Is it because you only hear half of the conversation? or do people talk louder on the phone?

I just wish they had telephone free rooms on campus, like there used to be smoking and non-smoking rooms.