How does Sirius tell my radio to work or not?!

How does Sirius tell my radio to work or not?

So I've got Sirius satellite radio in my car. It came with 3 free months. It's been beyond 3 months know, but the service still works. How does Sirius tell my radio to activate and terminate the service?

Does my radio send a signal to Sirius? If not, what if the radio isn't powered up when the satellite sends those signals, especially the terminate signal?

With Cars it works differently, most of the time they say 3 months free but somtimes they will give you 6 months so you will come and join. Sirius knows you are using the radio, if you leave your car on long enough it might update and you will loose the service.