Abstract: A circuit and a computer program for enabling the monitoring of a plurality of switch settings at remote sites by central telephone exchange equipment through regular telephone lines is described. To compensate for slow drift variations in line impedance, the impedance of each line is continually calculated from the measurements, and any small change is used to update a line impedance correction factor, while any large instantaneous change is used to update the data counter. Telephone exchange battery variations which may be large and instantaneous, are compensated for by measuring the battery voltage immediately prior to measuring the line impedances, and correcting the line impedance readings with a battery voltage correction factor. Other types of temporary signal interference which are frequently present in telephone switching equipment are identified and rejected by applying consistency criteria to multiple readings which have been corrected by the aforementioned method.