sorry, i didn't even notice you had said that about the drop in max attainable rate when lifting the handset. Definitely worth getting to the bottom of that. I don't know anything much about line faults, HR joints, etc.. Though the filters/splitters in the faceplates do sometimes fail. And of course there's another splitter/filter at the other end (on the linecard in the cabinet). That potentially could fail. Doesn't seem very likely though.
I have experienced dodgy external pole top connections AND failed filtered faceplates.
The symptoms were:-
Many random disconnections due to weak/corroded cable connection(s)
SNRM & Attainable rates dropping substantially (sometimes enough to cause a disconnection) caused simply by picking up a telephone.
Dialling in always seemed to effect a temporary cure that lasted from a few hours to a couple of days.
These issues were always more apparent during warm/hot & dry weather conditions & less apparent during wet & cold weather conditions.
It took around 11 months & many engineer visits to finally get these intermittent issues fixed.
Without the benefit of graphical & raw data evidence, I am convinced my connection would never have been fixed as it was always classed as being "within acceptable performance limits" by BT (& initially my ISP).