This European Telecommunication Standard (ETS) specifies the stage 1 description of Completion of Calls to Busy Subscriber (CCBS) from the subscriber s and user s points of view; in particular: -the procedures for normal operation with successful outcome; -the action to be taken in exceptional circumstances; -the interaction with other GSM supplementary services; This ETS does not deal with the Man-Machine Interface (MMI) requirements, but makes reference to the appropriate GSM specifications. The charging principles applied to CCBS are outside of the scope of thisETS. This ETS is applicable to the digital cellular telecommunication system Global System for Mobile communications (GSM). Any interactions with other networks not dealt with in clause 9 are outside the scope of this ETS.