the reversal in the wiring to your telephone extension socket
Replace the cover to the phone socket and replace the cover to your BT master socket. Plug one of your BT TwinTalks in to your BT master socket then plug your second BT TwinTalk into your other extension socket. Look at the Ring LED on each unit. If you have successfully cleared the reversal neither LED will be illuminated.
Plug a phone into the BT TwinTalk at the extension socket you have just fixed. Lift the handset to make sure that you can hear a dial tone and that your phone is working when connected to this socket.
Now you can test your BT TwinTalks, or press the blue arrow to view these steps again.