Cellular coverage has now reached almost 90% of the world's population while geographical land mass coverage only about 18%. 82% of the world's land mass has zero cellular reception. If you are into ocean or mountain activities that take you further than a few miles offshore or away from cities you will likely be in one of these locations.
The world of satellite phones is now available to everyone and prices have never been lower. Devices available include: personal tracking devices that fit in your pocket like the SPOT from Globalstar. Another popular device in the US for both hikers, boaters and corporations looking for an emergency back-up solution is the Iridium 9555 satellite phone. It is a ruggedly built phone and not a toy to surf the web or text your friends. This phone will work anywhere you have a clear line of sight to the sky and built to withstand the some tough weather environments. The last phone designed for the remote business traveler is the World's First Satellite Cellular Smartphone TerreStar GENUS Smartphone (not available yet from AT&T). This phone is designed to switch between cellular and satellite networks as needed.
Satellite Coverage Maps for Cell Phones