I wouldn't trust the little printed antenna that goes on the back of your cell phone. Get something with a big antenna and an amp requiring a power source.

I have something made by Wilson Electronics. I don't know how good this brand is, but it has made a difference in a communication situation. However, it wasn't as big of a difference as I'd hoped. I was driving on a highway and I needed to make an urgent call. I still had to drive a bit before the signal was strong enough.

Just occurred to me: you can probably find more informed opinions on cell phone forums.

As for whistles, the Nitcore is so tiny -- smaller than a key -- there is really no reason why you shouldn't have one on your keychain, along with a keychain light like Doug Ritter's Photon. Maybe a multitool, too.