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Interesting question. I imagine the ones that charge you must be tied to the phone companies somehow since as far as I know charges appear on your phone bill (someone correct me if I'm wrong about that).
But the ones that don't charge any fees can probably just be setup with some kind of server. The first step would be to setup a method for SMS messages to be routed to the server somehow. At the moment, I can't think of a good way to do that, but some cell phone company seems like a good place to start. Let us know what you find.
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