Abstract: A data communication system configured to promptly determine the validity of establishing the line connection between terminals and a host computer and maintaining the line connection. The data communication system includes at least a plurality of terminals and a sender telephone number detecting/informing device, which are interconnected by way of telephone lines including the public switched telephone network for use in the caller number identification and display service. By registering in the host computer ID codes for the terminals and telephone numbers corresponding to the respective ID codes, and by subsequently comparing the telephone number, which is currently detected and informed by sender telephone number detecting/informing means, with those registered previously, the line connection is established and the data communication is initiated with the terminal only when the telephone number is found among those registered.

Abstract: A data communication system configured to promptly determine the validity of establishing the line connection between terminals and a host computer and maintaining the line connection. The data communication system includes at least a plurality of terminals and a sender telephone number detecting/informing device, which are interconnected by way of telephone lines including the public switched telephone network for use in the caller number identification and display service. By registering in the host computer ID codes for the terminals and telephone numbers corresponding to the respective ID codes, and by subsequently comparing the telephone number, which is currently detected and informed by sender telephone number detecting/informing means, with those registered previously, the line connection is established and the data communication is initiated with the terminal only when the telephone number is found among those registered.