As always, you need to be within range of a tower. And it has to be a tower with your carrier, or a carrier that has roaming arrangements with your carrier. And, the technology has to be the same. Some newer phones use a new technology and are not backwards compatibale with older systems.
But, if you are 100's of miles from a town, and there is no road, guess what the chances are that somebody spent money to put a tower up?