Abstract: Corruption of digital data on a telephone line to which a telephone test set is to be coupled is obviated by inserting a low pass filter circuit in the tip-ring connection path to the test set. The low pass filter provides a low impedance path for only voice band signals, while blocking signals that fall within the high frequency range of digital data that may be present on the telephone line. The low pass filter circuit includes inductors coupled with respective tip and ring leads and a controllably switched, shunt-capacitor circuit coupled across the tip and ring leads. Also, resistors that compensate for impedance variations are coupled in series with the leads and are controllably by-passed, when said telephone test set is to go off-hook.