I like the hand gesture, although I usually use one that might be considered somewhat more inflammatory.

Holding hand about the same height as mouth, approx 12 inches away from head with palm facing the loud talker, fingers pointing up and thumb pointing towards loud talker. Bring fingers down and thumb up to meet to simulate a mouth talking, open hand, close hand again. Sort of like you would play with a sock puppet on your hand.
Do this until they notice it and then continue to do this until they lower their voice, look away or end their conversation.

I have found that having a poker face while doing this works best. Don't show any anger or frustration, if anything maybe a slight eyebrow raise as if in expectation of their call ending.

By the way if any one has ever been driving in the DTW area (highway), in the left lane doing 55mph*, talking on your cell and been passed, on the right, by someone doing the above... that was probably me.


*Highway speed limit in Michigan is normally 70mph.