Cellular phones use small base stations named "cells". When you move out of range from one cell, another cell takes over. Because of cost and logistics, cell towers aren't placed in extreme rural areas, in the middle of oceans, or in third world countries for the most part. Satellite phones don't rely on cell towers. The signal goes to a low orbiting satellite in the sky, so it will work where normal cell phones wont. Cell phone use terrestrial sites located on the earth and usually only get reception in a certain are. Satellite phone use orbiting satellites and they usually get reception all around the world.