This differs per provider. With us, it means that your SIM card and therefore your mobile phone are behind the switchboard. So everything on your landline also works on your mobile - if you want it to. Call forwarding, call waiting, transferring calls, picking up calls and 100 other functions. It all works perfectly with any mobile device without apps and complexities. PS We do have a mobile app (iOS/Android), but you control your account with it so you don't have to log in online. Very easy.