If a student's phone rings whilst I'm lecturing I stop talking and tell the whole class we're going to play 'name that tune'. Strangely enough, after about week 2, I have no problem with people accidentally leaving their phones switched on in class.
Who are the people who abuse cell phones anyway? A country's president needing to be informed of a worsening diplomatic crisis? A transplant surgeon being told a donor heart has been found and he must get to the hospital at once? Nope - it always seems to be some tedious little prole with no...