My trip in August had rented one from Cebia and we attempted to use it a couple of times to change our shuttle pick up time. We first tried at Granite Park with no luck at all. Then we tried to use it at Pierce Ferry and got signal for about a minute or two.
All in all I probably wouldn't get a satphone again. Instead I'd have a radio capable of aircraft frequencies and a personal locator beacon (406 Megahertz).