I do a Tesco shop on line for some elderly ex neighbours who telephone me on the land line with their list. What with the lady of the house going "Ay?" because she is very deaf and me going "Ay?" because of the awful crackling on the phone we take rather longer than we should to accomplish anything. .....HELP!!!
I suppose the first thing to do is establish if it’s your end or her end, does she have a problem with calls to other numbers apart from yours, or do you have problems with other calls or just from this neighbour ?
Once established which end has the problem, the telephone instrument at that end needs to be eliminated as the possible cause of the noise, this is usually done by plugging a different , known fault free , preferably corded, landline phone in the master socket test port of the lines with the problem.
If the phone is given a clean bill of health ( because the noise continues using the ‘new’ phone in the test socket, that person ( you or your neighbour) report their line as noisy to their landline provider, if the phone is the culprit, buy a new phone
You have posted on a BT Consumer forum, are both yourself and her both BT customers , or just one of you, or even neither ?