1. An apparatus for supervising the activities of numerous communication circuits of a telephone system as determined by respective call monitors, comprising: a plurality of signal detectors each including a multiplicity of sensors forming groups connected to respective call monitors for receiving output signals therefrom which are indicative of the state of energization of the supervised circuits, said signal detectors emitting individual output frames sequentially combining a predetermined number of bits derived from samples of the output signals received by the respective sensors; a data-input unit including serializing means connected to said signal detectors for receiving said individual output frames thereof and synthesizing therefrom a comprehensive frame with a constant number of time slots accommodating a multiple of said predetermined number of bits, said unit further including timing means for establishing a recurrent scanning cycle divided into several subcycles, said serializing means including switching means controlled by said timing means for selecting different combinations of output frames for assembly in said comprehensive frame during different subcycles, certain of said output frames derived from circuits in need of more frequent sampling being included in a plurality of subcycles periodically recurring within a scanning cycle; and processing means connected to said unit for receiving said comprehensive frame in each scanning cycle and evaluating the bits contained in respective time slots thereof.