Looks more like you need a NTE5 ADSL Filter faceplate (if you can conveniently run the modem from there)
http://www.amazon.co.uk/s/ref=nb_sb_...ctronics%2C218
Or an I-plate (ring wire isolater) if you can't
http://www.amazon.co.uk/s/ref=nb_sb_...%2Ck%3Ai-plate
It's the unbalance from the ring wire (which is not needed as microfilters provide their own ringer connection) that often degrades ADSL signal when extension wiring is connected.