Well, you could take out a pen and paper and start jotting down everything he says....

or there's the old standby:

Pick up your own cell phone (turned off will do, thumb and pinkie will do too....)

Scream into phone (hand, whatever)
HELLO?? WHAT'S THAT?? I CAN'T HEAR YOU, I'M SITTING NEXT TO SOME A$$HOLE WHO IS SCREAMING INTO HIS TELEPHONE.

It's even better if you start repeating details-

YEAH, HE'S TALKING ABOUT WHETHER OR NOT TO BUY XXXX

or

HE SAYS HIS MOTHER JUST PASSED A KIDNEY STONE AND...

That'll get his attention