The present invention relates to a device (108, 109, 110) comprising an interface to at least one active subscriber line (112) and an interface to at least one inactive subscriber line (111). The device (108, 109, 110) comprises means for transmitting a first signal on the inactive subscriber line (111) and means for synchronously measuring crosstalk (130; 201, 203, 205, 207) induced into the active subscriber line (112) by transmitting the first signal on the inactive subscriber line (111).