There are over seven billion cellphone on earth. In some nations, they are the dominant form of telephone communications. However if you must be able to place a phone call from literally anywhere (outdoors) the phone to have is a satphone.
Cellphones communicate with an antenna and transceiver (transmitter/receiver) normally located on and at the base of a tower. The tower has a defined area (cell) that it serves. Typically multiple towers are placed so that a call can be seamlessly passed off from one cell to another without interruption as the cellphone moves. Because the tower and associated equipment is expensive and limited to line-of-sight communications with the cellphone, areas with limited population density often have no cellphone service. Needless to say, the vast areas covered by oceans or large lakes lack cellphone coverage.
Portable phones are also designed to communicate with orbiting satellites. There are two types of satellites used with satphones, the geo-synchronous typified by INMARSAT and the low earth orbit (LEO) such as Iridium.
Celestial mechanics requires that the geo-synchronous satellites be placed 22,236 miles about the equator. Only three satellites are required to cover the entire earth (between 70° north and south). The area between 70° and 90° at both poles is not reachable. INMARSAT was originally designed to provide communications to ships at sea. The original system required rather large movable antennas and equipment mounted on a ship. Portable phones are now available.
The LEO satellites have north-south orbits at about 400 to 700 miles altitude and literally provide service over the whole earth. There are currently about 72 Iridium satellites, 66 active and 6 spares. The system works like the cellphone system, only in reverse, as one satellite disappears over the horizon the call is transferred to another one, if possible. The Iridium satellites have the ability to link a call across several satellites to a ground station if the originating satellite is not directly connectible to a station.
The five billion dollar Iridium system went bankrupt within a year of starting. Its owner, worried about liability, was planning to have each satellite drop out of orbit into the Pacific, but it was bought for about 25 million dollars as part of a rescue which included the US Department of Defense. The US wanted to preserve the system for use by the military (and, perhaps, spies) anywhere in the world. The military has its own ground station dedicated to secure communications.
Iridium satphones are used all around the world, including arctic explorers and TV live trucks roaming the wilder parts of West Virginia. Iridium satphones not much larger than a cellphone are now available.