I also had this once except this was an 'engineer' early VDSL BB install (I don't think they do these now for VDSL).
In anticipation of VDSL I had privately obtained the latest type of official OpenReach filter.
I wired the customers modem/router directly via the unfiltered block in the NTE and the then-PTSN Webway and phone to the filtered part.
F**kwit engineer comes along, disconnects all wiring punched down at NTE, tells customer they need to move phone and DSL to lobby.
Webway PTSN goes down, I get called out to find the above
Reverted to how it was and never had an issue since. Well, WW PTSN long gone now but broadband is fine