Guess Who's Calling?

I can't quite decide whether this is a good idea or not. There's a lot of advertising going on about it, at least. See, one of the cell phone companies came up with the idea of offering a small yellow box, with a ring for fastening it, and a tab for pulling. It's intended for children -- fasten it to your school bookbag, and if you think you are in danger, yank the tab!
( in case of emergency, pull here? )