Know where you are primarily going to be calling to or from. Then choose a provider of satellite telecommunication services. Some of the big players in this field are Iridium Satellite, Globalstar USA, Inmarsat, and Thuraya.
Iridium Satellite System is a truly global system, available anywhere in the world. With their system of 66 linked LEO (low earth orbiting) satellites, Iridium can go "where no man has gone before." Iridium started with 77 satellites, hence the name iridium, after the 77th element. When it comes to commercial application of satellite phones, Iridium is the largest, selling its services through partners throughout the world. Some industries making use of this system are mining, forestry (picture yourself in the midst of the primeval forest...talking to cousin Bennie), oil (riggings in the middle of the Gulf), the military and aviation.
Globalstar goes almost where no man has gone before, providing service to more than one hundred countries and some oceans. They also use LEO satellites, returning the signal to earth and a gateway and then to the land locked phone service. As you can imagine, they are expanding and will more than likely have the same coverage as Iridium soon. They offer internet connectivity, GPS capability, SMS (aka short messaging service) and the more plebian call forwarding. Globalstar's CEO, Jay Monroe lives in New Orleans and anticipated the issues Katrina might pose...cell and landline communications down...and beefed up their power for an anticipated hike in satellite phone usage. It was prescient, as Globalstar shipped over ten thousand phones to the disaster area to facilitate communication. Satellite phones were the only operating phones in the early days after the disaster.
Inmarsat will help you communicate if you are anywhere in the world but at the Poles. This company's geostationary satellites make possible phoning as well as communicating over the internet and transferring data. Inmarsat uses retailers called Inmarsat Partners to sell their services all over the world. They are very involved in working with air traffic control systems as well as with the GMDSS (Global Maritime Distress and Safety System) to prevent maritime disasters.
Thuraya's goal is to provide quality satellite and data phone connections whether in the concrete caverns of NYC or in natural ones in the Grand Canyon. This company works closely with local cell companies all over the world to make satellite phone to cell phone communication flawless. The idea is to maintain a connection with your cell phone through your satellite phone. You get phone service, data transfer, even faxing with Thuraya.
Look on the web for the best deal and package for you. www.satellitephone101.com lists and compares the big four's services and capabilities.