"This standard defines the stage two of the Call Waiting (CW) supplementary service for the pan-European Integrated Services Digital Network (ISDN) as provided by European public telecommunications operators. Stage two identifies the functional capabilities and the information flows needed to support the stage 1 service description. The stage two description also identifies user operations not directly associated with a call (see CCITT Recommendation I.130 [1]). This standard is specified according to the methodology specified in CCITT Recommendation Q.65 [2]. This standard does not formally describe the relationship between this supplementary service and the basic call but@ where possible@ the information is included for guidance. In addition@ this standard does not specify the requirements where the service is provided to the user via a private ISDN. This standard does not specify the requirements for the allocation of defined functional entities within a private ISDN; it does however define which functional entities may be allocated to a private ISDN. This standard does not specify the additional requirements where the service is provided to the user via a telecommunications network that is not an ISDN. The CW supplementary service permits a user to be informed of an incoming call (as per basic call procedures) with an indication that no interface information channel is available. The user then has the choice of accepting@ rejecting or ignoring the waiting call (as per basic call procedures). The CW supplementary service is considered meaningful when applied to the telephony teleservice and the speech and 3@1 kHz audio bearer services. Furthermore@ it may also be applied to other circuit-switched services. This standard is applicable to the stage three standards for the ISDN CW supplementary service. The term ""stage three"" is also defined in CCITT Recommendation I.130 [1]. Where the text indicates the status of a requirement@ i.e. as a strict command or prohibition@ as authorisation leaving freedom@ as a capability or possibility@ this shall be reflected in the text of the relevant and stage three standards. Furthermore@ conformance to this standard is met by conforming to the stage three standards with the field of application appropriate to the equipment being implemented. Therefore no method of testing is provided for this standard."