Why Don’t People Mute Their Phones?

I had a brand new cell phone. I had had no time to play with it before I had to run to a lecture in the university I was studying in. In the middle of interesting lecture someone’s phone started to ring. It rang and rang. I was annoyed. Why doesn’t the owner of the phone mute it? After a while I understood it was my own phone. I did not recognize the ringing tone because I had not selected it by myself, and at that moment I did not even remember I had a new phone.