the reversal in the wiring to your telephone extension socket
Remove the wires. You may have screw terminals on your socket, or Insulation Displacement Connectors (IDC) which will require the use of an IDC tool to connect the wires. A disposable tool is available from most phone shops, or DIY stores
You can buy a low cost IDC tool from most phone shops, or DIY stores, or wherever you can buy telephone extension wiring kits.