I believe I just thought of a temporary cure for these loud talking cell phone idiots.

Get YOUR phone out and get right up next to them (and I mean right up next to them) and start blathering LOUDLY into your phone. Ignore them if they give you 'the look'. If they move away, move with them. Eventually, they will stop and ask "Do you mind?"

Respond "Not at all, do you?" and continue your 'conversation'

I am going to try this.

"Never underestimate the power of human stupidity" - Robert Heinlein