Crackling noise from bt landline

System

Been there, done that, got it sorted! Dare I suggest you read through this thread of mine where much help is offered, that way you could do a process of elimination.

https://forums.moneysavingexpert.com/discussion/4947701

But, despite trying removing lid of bt box and using the cable directly, I have come to the conclusion that it might have been static caused by modem on router (or visa versa) and now they are separated, it has cleared up.