Great to hear that your enjoying the savings and benefits that using VoIP brings...
So you have an old (unused) Virgin Media NTE5 Master socket in the same room as your OBi202 which in turn feeds an extension in another room, and you want to use that extension socket to connect a second phone to your VoIP setup? No Problem!
You say that "virgin have recently disconnected master socket from their box outside" but to prevent damage your OBi202 (should Virgin ever reconnect your line) what you need to do is remove the Virgin Media telephone cable feed coming in from the outside from terminals A and B on the rear of your Virgin Media NTE5 master socket. Don't rip out the feed completely, just remove the two cables from A and B:
Once removed curl them up and leave them lose inside the back plate. That way, should you ever need the Virgin Media line reconnecting all the engineer need do is reconnect them.
You now need to connect your OBi202 to terminals A and B on your old Virgin Media NTE5 master socket. To do this you're going to need one of these:
Simple cut off the BT plug to expose the internal wiring and connect the two center wires to terminals A and B on the NTE5 master socket. To help work out which pair to connect to your master socket, use the following diagram:
Line 1 0v from the OBi202 Phone 1 port would be wired to the A leg on the NTE5 socket.
Line 1 -48v from the OBi202 Phone 1 port would be wired to the B leg on the NTE5 socket.
Plug the RJ11 into Phone Port 1 on your OBi202 and power it up...
Your extension cable should be coming off connections 2,3 and 5 on the NTE5 front face plate like so:
And then in turn connecting to 2,3,and 5 at the extension point like so:
You should now have a dialtone on both the NTE5 master socket and extension socket.
Just for the record, when connecting your OBi to your master socket, you no longer require a RJ11 (US) to BT (BTS) telephone plug adaptor with ringing capacitor
as the master socket already contains one.