Data communication system and medium for storing communication control program

A data communication system, capable of making data communication with a terminal unit of a caller via a communication apparatus which comprises connection instructing means for instructing to connect a line for transmitting/receiving signals and line control means for connecting the line with the terminal unit in response to the instruction from the connection instructing means, the data communication system comprising:

• request judging means for judging whether or not the terminal unit is requesting data communication on the basis of a call signal given from the terminal unit of the caller;
• transmission/receiving control means for data communication, capable of controlling transmission/receiving of signals indicative of data based on a communication protocol preset for the data communication;
• and
• communication control means for causing the line control means to connect the line when the data communication is requested in response to a result of judgment of the request judging means, for comparing authorization given from the terminal unit via the line with reference authorization defined in advance and for causing the transmission/receiving control means to start to control the transmission/receiving of the signals only when the authorization coincides with the reference authorization.