An impedance blocking filter circuit is provided for use in telecommunication systems for interconnecting between incoming telephone lines and customer's terminal equipment so as to unconditionally block impedances above .[.20 KHz.]. .Iadd.a desired frequency range .Iaddend.due to the customer's terminal equipment from .[.an ADSL.]. .Iadd.a DSL .Iaddend.network unit and/or home networking interface unit. .[.The filter circuit includes first, second, and third inductors connected in series between a first input terminal and a first common point. A first resistor has its one end connected also to the first common point and its other end connected to a first output terminal. Fourth, fifth and sixth inductors are connected in series between a second input terminal and a second common point. A second resistor has its one end also connected to the second common point and its other end connected to a second output terminal. A capacitor has its ends connected across the first and second common points. In other aspects, the filter circuit also includes switching means for eliminating shunt additive capacitance, correction circuit means reducing significantly return loss, and switch suppression circuit means for eliminating transients..]. .Iadd.In one exemplary embodiment, the filter circuit is adapted to block impedances above 20 KHz and comprises a series of inductors disposed electrically between respective ones of first and second input terminals and output terminals. First and second switches responsive to loop current are disposed in series with respective capacitors between the series of inductors..Iaddend.