A twisted-pair telephone line is connected to a conventional telephone via a low-pass filter and equipment for receiving signals in an upper frequency range, e.g., 25-1000 kHz via a high-pass filter. The low-pass filter is primarily passive (to allow passage of line power, ringing and signalling voltages)...http://www.google.fr/patents/US5623543?utm_source=gb-gplus-shareBrevet US5623543 - Two port signalling voltages filter arrangement
