I bought a corded phone aspecialy to use in the test socket and did the quiet line tests and it was evident on a few of the tests but as I said its very intermittent, we dont really use the land line much as our mobiles are unlimited calls, I have 2 bt cordless phones and its been on both of these at times its been present
I am on a long line in a rural area and they have been pulling fibre cables for quite a while in the underground ducts , the new cabinets have just been installed and just waiting for them to connect up,
I only have about 100ft at the most of bt cable in the open its all under ground then , this noise only staryed a couple of months ago and may have coincided with the cable pulling
