Connect BT phone to Virgin socket

We've switched phone from Virgin to Sky/BT, we've got socket V (Virgin) in
living room, which works & connects to the extension upstairs plus a second
socket B (BT) at the opposite end of the room which has now been activated
and is working. The two sockets B and V have a cable running between them,
but this has been disconnected at the V socket by the Virgin bloke, and he's
connected 2 Virgin wires inside the box. Basically, it was all BT at some
point, but Virgin appropriated this BT socket for their own line which comes
through the wall from outside.

Now I'm trying to rewire socket V to get rid of Virgin phone line and
connect this socket via the existing cabling to the working BT socket, but
obviously the insides of the boxes are different, and I don't know how many
wires I need to connect, and which ones. It only has 2 wires connected at
present to the terminal with 3 connectors; (the Virgin line is a white wire
with green dashes and a green wire with white dashes with the middle one
empty), but trying the same 2 colours from the BT line doesn't work. The
sockets are different so even the terminal numbers don't correspond.

Does anyone know what goes where, or can point me in the direction of a
guide please? All I know is that it *used* to work fine until Virgin came
along and buggered with it.