Not sure I understand what a 'splitter' is. The I Plate is an additional piece of kit which I think you have to buy from BT. I have a NES 5 faceplate but no 'I Plate' as I simply didn't connect the orange wire : achieves the same thing. The better result from the
test socket points to room for improvement in your internal arrangements
Not too sure that isolating the bell line at the master socket on it's own is 100% the way to go [unless it's the
ONLY socket] as any other bell lines around the property will be influenced by spurious emf and the filter, now disconnected, wont be able to screen any of those away down the BT plughole to presumably some earth someplace...