I have recently moved into a new apartment and I have rewired a couple of phone sockets as the had clearly been messed with before. While decorating I removed a blanking plate in the living room and found phone wires behind. I believe this might have been an optional upgrade when the flat was built, but they obviously wired them anyway and just put the socket on.
Behind this plate are 2 wires. One has a white sheath and looks like all the others in the flat. The other has a grey sheath and is different. As far as I can tell this is the last socket in the chain.
Where could the grey one go?