The faceplates provide a level of RF filtering but the MK4 are troublesome, as they provide no REIN (impulse) protection.
MK3 where the best but are rare and may not fit you face plate housing
I created wiki sometime ago when I was having issues and on some faceplates they separate neatly phone from data signals, other countries use a splitter (microfilter) to achieve the same but in UK we are posh, enjoy 
Behind the faceplate is the test socket, use that .
https://uktechhub.com/forums/topic/bt-master-sockets-and-face-plates-wiki/
In a completely sane world, madness is the only freedom (J.G.Ballard).
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