At conferences I attend, there will always be somebody whose phone rings loudly. The same happens in meetings and in class. It has probably happened to you. It happens to me every few months. Very embarrassing.
Sometimes organizers remind people, don't forget to turn off your phones. But how many times have you reached for your phone when somebody else's phone rings, just to check if yours is off?
Here's the idea: All people need is a quick 'flash' reminder to check their phone. It need only take a couple of seconds. When introducing a session, the organizer could flash a slide for a few seconds with: "remember to check your phone is on vibrate!" The speaker doesn't have to say anything. Everybody knows what to do; hardly anybody wants their phone to ring.
Of course the better answer is to have calendar applications build in vibrate mode automatically, as I discussed in my previous blog post, but we may have to wait a while for that.