I think that the forwarding is set in your account at the carrier level (like voice mail). I'm sure someone more knowledgeable can verify or dispute this. SkypeIn gives you a number that ppl can use to call your Skype account. You chose from a list of numbers for a given area code. Skype also provides call forwarding. Not sure if this would apply to your situation, but when I am in Australia, my (Australian and US) SkypeIn numbers forward to my AU number. When in the US, they forward to my US (T-Mobile) number. I just update the forwarding settings before I hop on the plane and switch SIMS on arrival.