Abstract: A telephone station instrument add-on control subsystem with selectable addressing and ringing response delay techniques enhances any conventional instrument by providing added functionalities to ordinary telephone subscribers. When one or more of this enhanced station instruments are used on the same telephone line, a distributed single line PABX is formed. The formation of this system is based on the continuity of a pair of ordinary telephone wires. The specific routing of this wire pair does not affect the characteristics of the service features. A special case of this distributed system is to provide local dialed intercom services for communications among locations within an isolated area. A simplified version of this invention can be applied to improve the efficiency of data communication in a Local Area Network whose terminal information speed is comparable to the channel bandwidth of the transmission medium.