OP, I don’t think most of the above posters understand just what it is you are doing. I do, having recently fitted a ADSL faceplate into my BT NTE5 master socket.
I take it you are comparing the NTE5 test socket direct from your BT external line with a ADSL filter plugged in, and without any internal wiring connected, with the ADSL faceplate plugged into the test socket, again without any connection to any internal wiring. So I guess it must be the faceplate. Which make are you using? Mine is by ADSLnation, which had good reviews. I could have got one for half the price elsewhere.