Abstract: Each station of a switched telephone network voiceband data communication system includes a data set and a telephone set. The data set includes primary signal processing circuitry which performs the real-time signal processing of the data set under the control of a microprocessor-based primary controller. The latter also controls connection of the data set and telephone set to, and disconnection of same from, the telephone line. A portion of the frequency spectrum of the telephone line is allocated to a secondary channel over which a secondary controller within each data set communicates test, health and other information to a counterpart within the other data set. Line functions, such as the switching of a station into and out of the data state, talk state and disconnect state, are not executed at least until a line control message indicative of the line function in question has been communicated to the data set within the other station via the secondary channel.