Have you tried the simplest test of all, the Phone Line Quiet Test?
If not, plug the simplest, CORDED phone you have, into the Master socket (removing all other connections temporarily), with an ADSL Splitter dongle included.
Dial 17070, selecting Option 2 "Quiet Test"; and listen.
At best, you should hear "nothing".
The noises can be various - "snap, crackle, pop", humming, tones etc, intermittent or continuous etc.
If you do hear any noise, report it to your PHONE Provider, who may be your BB provider; or some other party.
Do NOT mention Broadband.
Also try to track your Phone Line physically in principle, particularly as you have indicated that it is lengthy, so likely to have several joints etc in it.
Get some idea of the likely Phone Line Length.
You may be able to get this by using the BE BRAS Test; but unfortunately it does not work for all lines-
If it does work and also provides a Cabinet Number - make a note of this.
Also get some idea of the following two distances-
1) Point-to-point from the Exchange to your house
2) The (main) road distance from the Exchange to your house
If you can locate the numbered Cabinet, does it lie beyond your house relative to the Exchange, or closer.
Obviously there is little that can be done to the routing of your phone line (other than repair/remaking) of joints; but you will have a better idea of where it runs and be better able to assess where problems may arise.
In my own case, the cabinet is about 25 metres away directly across the road - BUT "my line" from the cabinet to my house is about 125 metres long, as it goes via a small, inconspicuous, jointing box about 50 metres away at right-angles to the "obvious" straight line route.
Strangely, the cabinet is directly outside a neighbours garden, so apparently shorter; but the records show that the neighbour's phone line is about 1340 Metres long!
So where does it wander to locally?