The present document defines the stage one 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. Stage one is an overall service description from the users point of view (see
ITU-T Recommendation I.130 [1]), but does not deal with the details of the human interface itself.
The present document defines the interworking requirements of private ISDNs with the public ISDN.
In addition, the present document specifies the base functionality where the service is provided to the user via a private
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 but does include interworking requirements of other networks with the
public ISDN.
Interactions with supplementary services not listed in clause 7 are outside the scope of the present document.
Charging principles 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 telecommunications services with the following
exceptions:
a) call 2, of the videotelephony teleservice (see ETS 300 264 in bibliography);
b) all other circuit-switched telecommunications services requiring the use of more than one B-channel.
The present document is applicable to the stage two and stage three documents for the ISDN CCBS supplementary
service. The terms "stage two" and "stage three" are also defined in ITU-T Recommendation I.130 [1]. Where the text
indicates the status of a requirement (i.e. as strict command or prohibition, as authorization leaving freedom, or as a
capability (...abbreviated)