Iridium is the only satellite phone provider that will give you complete global coverage which includes all oceans, airways as well as the north and south poles. The constellation has polar orbits covering every spot on the planet.
The Iridium constellation includes 66 low earth orbiting satellites. The satellites are cross linked and has multiple in orbit spares. Iridium has the largest commercial satellite constellation in the world.
To make and receive calls on a handheld satellite phone you must be outside away from large structures and trees with a 80% view of the sky.