If the phone and the broadband are OK both in the NTE5 test socket (via a dangle filter) and on your new extension socket using a corded phone and an ethernet cable to a computer, at both locations, then your house wiring should be fine
Regarding the filtered voice connection to the extension socket: I was just wondering whether you actually needed a phone there, but it appears that you do since the dual socket is now more conveniently located near an electric socket.
See how it goes with the "training period" and if there are still problems after 10 days or so perhaps you can post your router stats and also do a quiet line test with a corded phone from the test socket.
Edited by 4M2 (Sat 24-Aug-13 12:46:19)