Yes, it’s an extra charge to use the network to send and receive messages. But what happens is when your phone would normally lose a signal entirely, it will be able to pick up the weaker signal from the satellites as long as you can see the sky. So the only way to avoid it would be to go underground or where you can’t see the sky anyway.

Technically, the signal from the satellite is available for everybody, even in, say, densely populated urban areas, but the signal strength from regular towers swamps it, and so it doesn’t pick it up. But when you are in a rural area with no other signals, your phone can hear the weak signal from the satellite.