You pay for the service yearly. $99
That gives you the 911 and messaging.

They have a feature called "Tracking" that allows you to constantly send your location every 10 minutes. This is displayed on Goggle maps so you can be followed live, or later look at your trip. Thats $49 more.
I have it but won't renew Tracking next year. Its cool but not really necessary. Maybe if I was racing BITD.