While you're probably correct in general, that's not really always a valid assumption.
At small venues, it's quite possible to hear an annoying cell phone ringing. I was at an Indigo Girls concert recently, and Lucy Wainwright Roche opened for them (really good, and hilarious, by the way). During one part of her performance, she was speaking to the audience (small venue, remember?), and someone's cell phone a few rows back from the front started ringing. Lucy heard the ringing just fine, and asked the person to go ahead and answer the phone so that we could all talk to them. It was pretty darned funny, and hopefully quite embarrassing for the culprit.