THIS is exactly why i don't think cellphone use should be allowed in flight.

that said, i more than once have stared at of these obnoxious guys, waited until his eye caught mine, and--with a pleasant expression--used my hand (palm down, moving from high to low) to signal that he should turn the volume down. it generally works, although it helps if you've exchanged looks with your fellow travelers nearby beforehand. when mr. obnoxious looks around, he sees others are backing you up