A new approach for an extension-line circuit of a private automatic branch exchange (PABX), which does not depend on a line transformer and high-power feeder resistors, is described. A matched pair of electrooptical couplers is used in place of the transformer; they provide high-voltage-protected signal transmission with stable insertion loss and good dynamic range. Power dissipation of the feeding circuitry has been kept low by feeding the telephone with a constant line-independent current. A new status and signaling-detection concept is also described, which yields fast and reliable detection of status changes even in the presence of large common-mode signals during ring condition. Elimination of the transformer, reduction of power dissipation in the feeding circuitry, and maximum use of integrated and hybrid circuit technology lead to a high packing density. Copyright © 1978 by The Institute of Electrical and Electronics Engineers, Inc.