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 the European public
telecommunications operators by means of the Signalling System No.7 protocol for the ISDN User Part (ISUP).
Stage three identifies the protocol procedures and switching functions needed to support a telecommunication service
(see ITU-T Recommendation I.130 [1]).
The present document specifies the additional requirements where the service is provided to the user via an intermediate
ISDN.
The present document does not specify the additional requirements where the service is provided to the user via a
telecommunications network that is not an ISDN.
Although the present document applies only to the international interconnection, the specification of functions, formats
and codes of messages and signals, and actions performed at originating and destination local exchanges are retained.
Formats, codes and procedures marked for national use are included for informative purposes for the international
interface specification. If these items so marked are supported within a national network and operators network, then it
is proposed that they shall be supported in this manner.
NOTE: In the case where a national signalling system behaves differently, the international gateway exchange is
to support both the concerned national and international network.
Charging aspects are outside the scope of the present document.
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 destination B becomes not busy.
The CCBS supplementary service is applicable to all circuit-switched basic services.