Abstract: A telecommunications system operable to connect terminal devices for exchange of data has control devices (ST, GST), an accessory control device (HS), and a memory (SP). The control devices effect addressed connections via hierarchical coupling fields (KF) to which terminal devices (T, PC, VP) and/or transmission lines (VL) are coupled, each by two user channels and one signalling channel. Routine transmissions to and from a telephone apparatus (T), computer (PC) or the like are effected by connection addressing codes transmitted on the signalling channel. For special functions or transmissions, a unique code is inserted in the signalling data, which causes the control device (GST, ST) to pass the signalling data to the accessory control device (HS). The accessory control device replaces the unique code with a connection addressing code if the terminal is authorized, and returns the signalling information to the control device, where it effects the desired connection.