The BT line and the NTL line will both terminate in seperate master sockets somewhere in your house. The house telephone wiring is then connected to that.

NTL won't fiddle with the BT socket because they're not allowed to since it is owned by BT and it's easier to just fit a new one anyway. If you ever decide to switch back to BT then the socket is already there.

They'll do all the work for you though so don't worry

- Trevor