RJ45 to RJ11 (phone connector)

rdp1980

Assuming you mean a UK BT socket rather than RJ11, what you need are two PBX Master adapters (one at the wall socket and one at the other end eg patch panel) plus one BT to BT lead (ie a BT plug at each end, male to male).

Plug one end of the BT to BT lead into the BT master socket, or rather the PHONE socket on the ADSL filter, and the other end into the PBX adapter.

Obviously your phone then plugs into the PBX adapter in the room you want.