Where to find the test socket and and how to connect to it

You'll need to ask the customer to find their master socket.

It's usually installed close to where the telephone line enters their property (sometimes this will be in the attic or a basement).

Most look like the one shown below: a split faceplate socket with two screws holding the bottom section in. Some older sockets are not split and the screws and socket will be in the middle.

Ask the customer if their master socket has a two part faceplate like the one shown above. If it has, they should remove the screws from the bottom section and gently pull it forwards - away from the main socket. Tell them to be careful not to disturb any wiring in place on the back of the bottom section.

If the customer has a test socket, it'll look like this:

Ask the customer to connect their router to the socket shown. Tell the customer that other sockets in their house won't work while they're connected via the test socket.