A method to detect a defective or missing filter at a customer's side between a first connection having a first frequency spectrum and a second connection having a second frequency spectrum and using the same physical transmission medium as the first connection between said customer and a central office equipment (DSLAM), comprises the steps of generating and transmitting at least one first type signal within said first frequency spectrum simultaneously performing quality measurements on said second connection in said second frequency spectrum, analyzing said quality measurements for thereby concluding the presence or not of the correct functionality and presence of said filter. An apparatus for performing the method is as well disclosed.