How to use the Test Socket & How your Internal Wiring should be installed
Since 1981 BT have been installing the NTE-5 Master Socket, this should be near the phone cable entry point in your home and the first and only box connected to that cable, the NTE-5 is designed with a removable faceplate, Secondary BT 'Extension' Boxes around your home are wired from this faceplate and should be wired using BT Grade CW1308 Twisted Pair cable as its high quality copper and the twists help cancel out interference in your home, the faceplate then plugs into whats called the "test socket" which is inside the main section of the NTE-5, this is so your internal wiring can be disconnected from the outisde so tests can be done without any internal wiring affecting the results, this is especially handy for broadband faults as internal wiring can cause problems with broadband.
When your using the test socket to test your broadband connection you should try and use a new or un-used filter and never use long extension cables to extend the reach of your router (infact this is best practiced normally, not just in test socket) because cheap phone extension cables tend to be untwisted pair and of low quality ultra-thin metals, this affects noise cancellation and degrades the signal, also the longer the cable the more chance of noise being introduced into the line, you should always use the short RJ-11 ADSL cable supplied by your ISP, or you can purchase a short Un-shielded Twisted Pair (uTP) or Shielded Twisted Pair (STP) RJ-11 cable which would be better as the supplied cable is not Twisted Pair.
Now if you have an NTE-5 master socket installed then its front is split into two sections, the lower section has the 2 screws on (as in image below), if your socket is not like this then you have an older LJU type socket which does not have a test socket and looks just like an extension socket, you can not test from a master LJU without having internal wiring connected (unless you remove the daisy-chain extension wiring yourself)
Your testing setup should look like this picture on the left (you dont need a filter in the faceplate itself whilst your plugged in the test socket, that one is there to show a new un-used one being used in the test socket) - the picture on the right demonstrates how your internal wiring should be setup.
If your plugging directly into the test socket for testing then once your all connected like the picture on the left (ignoring the filter in the faceplate on the left) you can turn on the router and gather the stats and see if there is any improvement.