Even Emma Watson makes the mistake you made while in a 200-person lecture hall course in college—not turning off her phone. In this case, she got a noisy phone call while being interviewed for television, and she was only saved by picking the best possible song for a ringtone. It's not very Hermione-like.
The interviewer says the ringtone is the best he has ever heard, and that he only respects Watson more because of it. "I'm glad that I'm forgiven because of Tina Turner, otherwise that would have been terrible," Watson replies. So if you forget to put your phone on silent before going into, say, a job interview, try making this your ringtone first:
That is a pretty damn awesome ringtone.