Connecting the missing ring wire to your telephone extension socket
You can buy a low cost IDC tool from most phone shops, or DIY stores, or wherever you can buy telephone extension wiring kits.
Check for a wire connected to Pin 3 of your BT Master Socket coming from the extension socket you are investigating. There may be other wires connected to Pin 3 from other extension sockets.
Identify the cable which is coming from the problem extension socket. Find an unused wire on this cable. The cable will probably have four wires, one of these will be connected to Pin 2, one will be connected to Pin 5 and there will be two spare wires (or one may be connected to Pin 4 - ignore this one).
Take one of the spare wires and connect it to Pin 3 on the BT Master Socket. You will probably have Insulation Displacement Connectors (IDC) which will require the use of an IDC tool to insert the wires. A disposable tool is available from most phone shops, or DIY stores.