How to find it
The first step is to locate your master socket; this is usually located where the line enters your building and has ‘BT’ or ‘Openreach’ written on the faceplate.
Unplug any equipment, then carefully remove the two screws on the faceplate using a small screwdriver to reveal the test port. If your master socket doesn't have a faceplate you can skip this step.
To connect to it, take the microfilter or DSL cable that should be plugged into your master socket and plug it into your test port.
How does this help?
The test port lives behind the faceplate of the master socket, so connecting to this should remove the possibility of any extension wiring affecting the quality of your service.