Abstract: Disclosed is a communication system which accommodates at least one central office network line and a plurality of extensions, and which is adapted to control line connection between the central office network line and the plurality of extensions. When a call from the central office network line is detected, it is determined in response to the call whether a control signal characteristic of data communication, such as a CST signal, is being sent by the other party's device. If such a control signal is being sent, a line to which a data processor is connected is selected and connected to the calling line. This makes it possible to connect an information processor and a telephone to a common central office network line, and to connect a call answered by another telephone to a desired information processor rapidly through a simple operation. Accordingly, it is unnecessary to assign a special-purpose central office network line to an information processor.