To put things into context.
Houses where I live are pre-war (the 2nd WW {a little like now}) and so I expect the underground cables are pre-war too, up to the FTTC connection.
We have had noise on the line on and off for thirty years.
We have replaced the DECT phone several times, mostly BT phones.
I have a Mk4 master socket, which has two connections, phone and BB.
There are no additional phone sockets = no other devices like Sky boxes etc. connected (usual reason the remove face plate)
The master socket (ms) has been replaced twice.
The cable from ms to wall outside has been replaced in the last ten years.
The cable from the wall to the telegraph pole has been replaced also in last ten years.
On seeing a Openreach Engineer down a hole next to the telegraph pole and the phone suddenly getting noisy, less than ten years ago we booked a call and the twisted pair running down the road was changed.
On BB speed dropping a lot last year including Upload dropping from 10 to 1 Mbps, Openreach visited again, the guy went above and beyond the call of duty and found that at one of the green cabinets down the road it still has "screw terminals" supposedly junked years ago, one of these was loose. {The guy got rollocked by his boss for this as all he was supposed to do was check the speed at my house}
The electrical/electronic noise as far as I know possibly exists but is unlikely to affect the phone (ok pulling rank here as I used to be an electronics engineer and moved to IT for a long while too)
I have tried to get help (from PN and OR) with the "noise" and have moved or eliminated possible sources, LED lights (yes dc ! but they get it a noisy way from 240v) DECT phones, microwave etc. far enough away and all mains leads have ferrite cores etc. Didn't think of the central heating pump. maybe it's noisy.
The noise on the phone line is odd, call another help line and the recorded message was clear as a bell, as soon as it switched to a real person the crackling started, noise is also at our end not the person we are speaking to which makes reporting the fault annoying as PN etc. have a perfectly clear sound.
Out of the possibly fifteen (could be twenty) visits we have had over 30 years no fault has ever been found at my home, albeit we have had all the changes above wires sockets etc. but 99.9% of the time the noise was never evident at the time the engineer visited, and it is possible that noise was evident to people at the other end of a call sometimes.
And.. As I have both phone and BB from PN I don't have to go round in circles when booking a BB call.
CONCLUSION
Eighty year old cables underground and obsolete connections in green cabinets probably don't help.
I have no idea if voice goes VoIP once at the green cabinet.
Openreach must waste a LOT of time on problems like this which would be totally eliminated with FTTP.
The government decision to let NTL (now Virgin Media) have a monopoly (cos we will go bust if we have any competition) on a fibre network some thirty years ago prevented BT/Openreach from installing fibre throughout the country = UK being years behind most of Europe and Asia in connectivity to homes and businesses.
One of the things that would get the UK back on its feet when we recover from the present problems is better communication for the whole country = countryside not just towns ! Put fibre in everywhere ! (and get Virgin Media to share their network as BT Have to ! )
And finally when I did get through it was remove the BT face plate and install a micro-filter (again)
