Abstract: A line interface unit for a digital local office for telephonic communication has a circuit connected to telemeter low-frequency signals around a line, alternating current, hybrid coupler and reinsert the same signals in the call path at the input to an analog-to-digital encoder that is coupled to a time division switching network. Each interface unit also includes circuits for multiplexing various line unit, digital, status signals with coded call signals and for demultiplexing line unit, digital, control signals from coded call signals. That network is utilized by an office common control for interconnecting various call paths with one another and for interconnecting the common control with the line interface units as required. The common control includes a time-shared digital filter for processing the telemetered signals for various office purposes.