The ISDN user-network interface is defined in the CCITT D-channel protocol recommendation (I-400 series recommendations), in which many kinds of communication terminals (e.g. faximile, telephone, teletex, etc.) can communicate through a subscriber line. The user-network interface is improved from the view point of effectiveness of terminal usage, provision of communication service, and, flexibility of system configuration, by providing a control terminal in the subscriber. The control terminal doubles as ordinary terminal equipment, and has a global control function for controlling other subscriber terminal equipment. The control terminal establishes and/or releases a local control phase in which a control terminal controls ordinary terminal equipment in the same subscriber to have them correspond to the communication with another subscriber using the means which belong to the ordinary terminal equipment inherently. The control terminal operates as an ordinary terminal apparatus when it is not in local control phase.