I wonder if the Main socket is exposed to the Rain/Damp? Or a very Cold Temperature?
Is her landline Noisy or Drops out/Fades/Faint? when the Weather is not good?
Fix, ring the BT fault line( think it's 150?) Or post on the BT site No need to write anything just tick the symptoms and enter your Phone Number. Will take a day or so to process, and you can track if the Job has been carried out This Service is Free Ring the 150 line again if nothing improves, can speak to an engineer