I keep my phone on silent, because I'm not fond of ringtones. The problem with them is that you get trained to react to a particular series of notes or sounds... and then you start hearing that everywhere. I'd rather not ruin my music by thinking my phone is ringing.
The trade-off, then, is to keep my phone on vibrate. This isn't usually a problem.
Except today, where my leg directly /under/ my phone has been twitching every 10 seconds or so, and it /feels/ like my phone is buzzing. o.o