I thought about it for a few minutes and I couldn’t come up with any foreseeable alternative for those way out in the sticks besides satellite phones. Nor much apart from more satellites to improve their use. Another point up above or offshore to receive and strengthen the signal, maybe? (Though every new point would introduce security risks, I assume.) I have an acquaintance who recently retired from a career in the Signal Corps, I’ll bounce the question off her (haha) and see if she has anything to add.