I've read it mentioned in other posts that the NTE5 adsl filtered face plate is the ideal solution for ruling out interference or degradation of the adsl signal from the internal wiring since it cuts out the internal wiring from the equation. The second best is the testing scenario of plugging a filter directly into the test socket and then the adsl modem into the filer using a short length of the RJ11 adsl cable. This however introduces the slight possibility of the filter or the wire or the connections on the wire causing a problem which can be mostly ruled out by trying different ones.
It occurred to me that a small custom cable could replicate the high standard of the adsl filtered NTE5 faceplate without the need to actually fit one. This suggestion is purely for convenience and the ability to test to the same standard as an adsl filtered NTE5 faceplate.
The cable would be a BT plug to an RJ11 plug, the BT plug goes into the test socket and the RJ11 into the adsl modem. These can be bought, but I'm not sure the quality of the wire used, so for a top quality version I'd suggest a custom wired cat5e cable with BT plug on one end and RJ11 on the other. A BT & RJ11 crimper and plugs would be needed and only 2 wires need to be connected ..... use a twisted pair and link them correctly ( centre two on RJ11 to 2&5 on BT as described here
http://www.wppltd.demon.co.uk/WPP/Wiring/UK_telephone/uk_telephone.html )
Anyway, I think this would give you a quick test option of the same high quality standard as an adsl filtered NTE5 faceplate.
Would the experts agree ?