Abstract: A telecommunication exchange which provides secure access by only authorized terminals to a computer connected to the exchange. The control processor in the central control unit of the exchange is programmed to provide a "call-back" procedure in respone to a request from a terminal for access to the computer. The calling terminal enters by tone or pulse dialing, the address (such as a telephone number) of the computer together with an identification code. The control processor compares such identification code with a list stored therein of authorized identification codes and authorized terminals addresses corresponding thereto. If the identification code is authorized, the control processor causes a confirm signal to be transmitted from the exchange to the requesting terminal signifying to the user of such terminal to release its line, after which a ringing signal is transmitted to the authorized terminal address corresponding to the authorized code.