The present document specifies the stage three of the Completion of Calls to Busy Subscriber (CCBS) supplementary
service for the pan-European Integrated Services Digital Network (ISDN) as provided by European public
telecommunications operators at the T reference point or coincident S and T reference point (as defined in
ITU-T Recommendation I.411 [6]) by means of the Digital Subscriber Signalling System No. one (DSS1) protocol.
Stage three identifies the protocol procedures and switching functions needed to support a telecommunication service
(see CCITT Recommendation I.130 [3]).
In addition, the present document specifies the protocol requirements at the T reference point where the service is
provided to the user via an intermediate private ISDN.
The present document does not specify the additional protocol requirements where the service is provided to the user via
a telecommunications network that is not an ISDN.
The CCBS supplementary service enables user A, encountering a busy destination B, to have the call completed without
having to make a new call attempt when the busy destination B becomes not busy.
The CCBS supplementary service is applicable to all circuit-switched telecommunication services, except the video
telephony teleservice involving a second connection.
Further parts of EN 300 359 specify the method of testing required to identify conformance to the present document.
The present document is applicable to equipment supporting the CCBS supplementary service, to be attached at either
side of a T reference point or coincident S and T reference point when used as an access to the public ISDN.