Sky boxes are notorious for this problem. If the noise goes away when the Sky box is unplugged from the line then the box is faulty. (Remote operators cannot test the box for line noise and so I have no idea what Sky are up to.) So just call Sky and tell,them to replace the faulty Sky+ box.
BTW unless you have multi-room (more than one Sky box in use for an extra £10 subscription) or PPV the Sky+ box does not need to be plugged in to the telephone line.
t
July 2008