You're somewhere where you really need to be quiet? I don't mean a library or theatre or anything (though I can imagine lots of phones going off in the middle of a show would be very off putting), but say you're unfortunate enough to be involved in a terrorist attack/ there's a crazed gunmen on the loose and you've found somewhere to hide.... and then an emergency alert is sent to warn you, and the noise tips off the guy you're trying to hide from??