Sometimes getting quieter rather than louder makes people shut up and listen.

I should probably clarify what this pub sing is like, since I've learned the terminology in the second etiquette thread. This sing is actually an invitational performance song circle with 8 to 16 people, and while it takes place in a pub, it happens in the back room with no audience present. Most of us have a couple pints in the course of the evening, but some of us don't drink at all, so it's not particularly rowdy, and its somewhat thematic -- older English pub songs being the focus.
So the guy who calls for the waiter does it in a room where the only other sound is me singing.
Blessings,
Barbara