I think the first thing I'd do is reply to their whistle with one or two whistle blasts of my own, so they know someone is hearing them. Then what Bee said, though depending on where we are, I might not send a send someone off for help by themselves - being a Scouter, I'm a big fan of the buddy system.

Also, since I carry a PLB when I think I might be out of cell phone range, that can call for help as a last resort.