That sounds really interesting but how does it work? From the website I cant see how it switches off my SIM and goes to their network. I would be worried I end

That sounds really interesting but how does it work? From the website I cant see how it switches off my SIM and goes to their network. I would be worried I end up with massive bill shock...and would I keep my number or need a new one? If it works as promised that's a great product so the marketing profession in me is disappointed with the website.