Is it, like mine, one of the original BT engineer fitted NTE5 faceplates (V 1.0) back in the days where you had no DIY option?
They work fine indeed with FTTC, well mine has since October 23rd! (and my speeds are exactly as the estimation)
You can buy replacements through Amazon. The filter in mine went duff a couple of years ago (originally fitted in 2001). The replacement I got from Amazon was the same make that BT fit (and had an instruction leaflet headed BT!)