Line errors are generally caused by induced line noise. Whilst the BT line itself can be susceptible to induced noise, 90% of the time the noise is being generated in the home phone wiring.
Extensions are almost always a problem for a good quality ADSL signal - in general, the longer the wire run between the master socket and the socket the router is plugged into, the worse the problem. Other things in the house can also contribute.
Fitting an ADSL faceplate to an extension socket is pretty much a waste of time as you're applying the filtering after the problem. An ADSL faceplate works best fitted to the master as this is then filtering out the extension wiring (where the problem normally arises). If you must have the router on an extension then you need to fit a faceplate to the master then run a dedicated extension, unfiltered, to the router.
To see what benefit you'd get, I'd advise running a temporary power extension to near your master and plugging your router into the test socket (hidden inside your master presuming you have an NTE5). You should then see an immediate change to your stats if your home wiring is the problem. What you're looking for is a jump in your noise margin.
