When cellular phones started becoming popular in the 90s most people were buying them for emergency usage only. Now cellular phones have become more of a standard. Everyone has one and may have discontinued their landline service because it is an unneeded expense.
Everyone has been in an area where their cellular phone does not have a signal which can be frustrating. There are many areas in the U.S. where there is no cellular coverage. A cellular phone is basically an advanced radio that must rely on cell towers close buy to receive a signal. No cell towers close by means no signal. The basic way cellular phones work is as you travel it receives a signal from a tower. The electronic serial number (ESN) is registered with the cellular network establishing a connection with that cellular provider. As you travel you will move out of coverage of that cell tower then if there is another cell tower you come in coverage range with, there is a soft hand off to that second cell tower not disrupting your service. When you make a call from a cellular phone the call goes to the cell tower to a landline then either to another cellular network or landline. The reverse is true when calling a cellular phone from another cellular phone or landline.
But what happens if cellular service is disrupted or overloaded. After 9/11 we can see how vulnerable we can be. After the twin towers were hit cellular and landline networks in New York were either disrupted or overloaded. Cellular and landline calls coming in and out were very difficult to make. After 9/11 many government agencies realized they must have emergency backup communication that does not rely on cellular or landline services.
Cellular and landline service are also disrupted by hurricanes. After hurricane Katrina cellular and landline communication was disrupted for weeks. Satellite phones played a key role in providing reliable communication where all cellular and landline networks were disrupted.
Satellite phone networks work when cellular networks are disrupted because satellite networks do not need to rely on the public phone system. Satellite phone calls basically go up to a satellite then down to a gateway that most likely will be out of the area of disaster. The call is then sent to its call destination which could be a landline or cellular network. The reverse is true when calling a satellite phone from a landline or cellular phone the call goes to the gateway then up to the satellite where it is relayed down to the satellite phone.
Satellite phone to satellite phone calls work differently when using the same satellite network. Calls are either handled completely by the satellite constellation when using the Iridium service never using a gateway or the public phone service. With the Globalstar network the gateway servicing the area handles the calls back and forth to the satellites never using the public phone system. This is why it is important for your emergency communication group to always use the same satellite phone provider.
Keep in mind even if you are using a fully functional satellite phone you will not be able to call a cellular or landline phone in an area where the cellular and landline service has been disrupted. This was very hard for many users to understand during the Katrina disaster. It is also important for a satellite phone user to be outside away from large structures and trees. A satellite phone user must have line-of-site to one of the satellites providing service to the phone.