A call control signal transmitted by the called data circuit-terminating equipment (DCE) to the calling data terminal equipment (DTE) to report (a) the progress of a call by using a positive call progress signal or (b) the reason why a connection could not be established by using a negative call progress signal. 2. A voiceband tone that is applied to the network interface (NI) by the customer installation (CI) to indicate the status of CI handling of a call attempt.