As Licquorice says, if you are on part fibre/FTTC/VDSL you need those incoming wires but what you can do is move your internal extension wiring from the master socket to the socket on the back of the SH2, to make the extensions work again.
I’d be surprised if that socket produced enough voltage/current to work an extension bell powered solely from it but, as I said, it’s not an item I have any experience with.
WSH, yes that is correct , if your are on FTTC or FTTP you can isolate the internal wiring and plug it into the back of the HUB by splicing a phone cable into a terminal block.
If you are on FTTP you must remove the dead access wires so they can't send external voltages/shorts into the HUB via the 431A
The YouTube video at watch?v=Id_KGXMcJHk at 17:35 he connects the old extension bell.
Another easier method to connect a phone/tone is using the free BT wireless Voice Adapter, youtube vid
watch?v=V2n1yJyXMak
I wonder if i can plug the old bell into a Voice adapter...mmmm
Scouser
As suggested in message 5