Abstract: Changes in a telephone subscriber line voltage are detected in a telephone set by supplying a voltage dependent upon the subscriber line voltage to a potential divider having first, second, and third tapping points, the second tapping point being between the first and third tapping points, and comparing the voltages at the first and third tapping points with a smoothed version of the voltage at the second tapping point to produce pulses at respective terminals when the line voltage is falling or rising. An algorithm is described for responding to the pulses to distinguish changes in hook state of another telephone connected to the same subscriber line from interfering signals. The arrangement facilitates remote release from hold and privacy indication functions for the telephone set.