Connect a corded phone into the working socket and dial 17070 and take option 2 the quiet line test, you shouldn't hear any crackling or hissing or any other sounds or noises, if you do then you need to contact the phone service provider and report it as a voice fault, don't mention the broadband issues.

HR joint on the line !.

I assume you posted that before reading the rest of the thread . In particular, Zarjaz's first two posts.

I had the exact same issue, I live in Warrington and was with ZEN.

I had countless engineer visits and even tried a second line still the same issue.

Only way I solved it was switching from ZEN (BT Openreach) to SKY LLU. Crazily ZEN actually advised me to try this as they said SKY has much better equipment in the exchange than Openreach.

No corded phones lying around I can test out at the moment, but I'll give that a shot when I can get hold of one.

Around £5-£7 from Maplin, Argos, B & Q and many other similar places. Always worth having one sitting in a drawer somewhere in the house.

I had this problem for over a year, dropped out even when no phone attached. After several fruitless visits from BT engineers I changed to a LLU line with Goscomb and cured immediately.
So that proved to me it was BT equipment at the exchange even though they said it was fine.