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Apple:

"When your iPhone detects that you’re outside of cellular and Wi-Fi coverage, it alerts you — on the Lock Screen, you get a prompt that you can send and receive messages via satellite. Otherwise, you get a notification to Use Messages via Satellite when you open the Messages app."

Looks like it's free, at least for the short term.

All the details here...

https://support.apple.com/en-us/120930
#Apple