Hasn't happened yet, but I guess it will, given time. (And probably in the middle of a seminar given by an eminent o/s professor, or such at uni). And yes, I wear it in my bra. Note to self - turn it on to vibrate mode when at uni.
I think I'd just do the reach down the top, pull it out, suspend or whatever, and smile sweetly. (Luckily classicists/ancient historians tend to be a wee bit absent minded, and probably wouldn't think anything of it.) If it had happened when I was performing (and I did do solos with a local choir), I'd just keep singing, which is what I did once when I forgot the words of an aria - just kept on making Italian type sounds, nobody noticed. And my accompanist always played loudly. If it happened before I started, I would have excused myself for a sec, walkied off stage, fixed it, and come back on. Just a part of the rich tapestry of life.
Infundo ergo sum.