Some places are quite rude about telling the audience to shut off their phones. In London they play a very loud recording of a phone ringing just before the performance starts. At the San Francisco Opera Gockley himself makes the announcement.
Politeness is just not working. The low key approach at Santa Fe resulted in the worst offense I have experienced. A very loud phone rang and rang with no attempt to stop it until voice mail kicked in. Bitch! Bitch loud and long! In this case I don't know if it could be heard on stage.
Second place goes to the Sacrificium concert in Munich: a phone rang in the middle of one of the arias. Cecilia reacted like she had been punched in the stomach and waited--certainly not calmly--until it had stopped before resuming. Her orchestra were completely with her all the way. She looked physically hurt.
You can't count on people to do the right thing. An audible announcement seems to work best. Or how about a large banner run across the stage that says, "shut off your damn phone."
I volunteer to perform this function. I'll come out in my plaid pants and say it in whatever language is required.