I recently had problems with my phone line as it comes in to the house. Everything looked fine, and my tinkering did no good at all.
I got a BT engineer in who, most unusually, appeared to know his stuff.
He simply shortened a couple of the leads and put them back in. It worked. Weird, huh! I always wondered why phone engineers left so much spare wires inside connection boxes.
He said that not-quite-complete connections that looked perfectly OK were a common fault.
Whilst not an issue for me, he added that another common fault was that wires that looked OK could have a break in them that could not be seen. His solution for that was to prove that a wire worked somewhere else, and then to replace it, making sure that complete connections were made.