A Skype adapter converts your regular phones to Skype dual phones, capable of making landline and Skype calls. Furthermore, in some models, such as AU600, B3G and B2K, you can forward Skype calls to landline calls and vice versa.
The last feature can save you big $ if you are on the road often. You can call your own Skype adapter using a landline and then forward the call to an international SKype call; or you can make a free Skype call to your own office from another country and forward the call to a local landline. Just imaginine the possibilities.