If you have a Sky Digibox it is important that this is connected through your TwinTalk. Your Digibox will have been supplied with a "doubler" connector. This doubler can be plugged in to the socket on your TwinTalk. You can then plug one of your extension phones in to the socket on the Doubler.
BT TwinTalk will not affect the operation of your Digibox, or your Sky Television service.