Problem: despite a room or bus or train's worth of dirty looks, the talker does not lower his or her voice.
Solution: Take out your own cellphone (if you have one. It's not essential). Begin your own loud conversation, but with the original loud talker. Provide the reverse end of their conversation. Eventually, they will notice the mysterious and not quite accurate echo of their conversation.
Expected results: anger, denial, embarrassment. Who cares, so long as they shut up. Warning: fistacuffs may result. Be prepared for potential violence.
Why this works:The talker does not intend to be rude; they are simply insane. Cell-phones cause temporary psychosis. This results from the brain's inability to be in two places simultaneously, causing the reality in the talker's head is more vivid than the actual, real, reality (i.e. mine. And yours, I suppose). In addition, the talker's optical cortex is disrupted and visual signals such as grimmaces, pointing, and lifted middle fingers are actually not seen. With sound and voice, you can actually enter the mind of the talker. Only by joining the talker in their madness can you cure them of it.