ISO/IEC 21991:2002规定了“QSIG”和“H.323”之间的信令互通，以支持企业电信网络（CN）内的呼叫完成补充业务。 “QSIG”是一种信令协议，在专用综合业务网（PISN）内的专用综合业务网交换机（PINX）之间的Q参考点运行。ISO/IEC 11579-1中定义了Q参考点。PISN为用户提供电路交换的基本业务和补充业务。QSIG在其他标准中有规定，特别是ISO/IEC 11572（支持基本业务的呼叫控制）、ISO/IEC 11582（支持补充业务的通用功能协议）和一些规定个别补充业务的标准。 ISO/IEC 13870规定了支持呼叫完成服务的QSIG协议。 “H.323”是一组信令协议，用于支持分组网络内的语音或多媒体通信，尤其是使用互联网协议（IP）作为其网络层协议（IP网络）的分组网络。H.323信令协议在IP网络中的端点之间运行，可以通过一个或多个网守间接运行，也可以直接运行。端点可以是另一个网络的终端或网关。H.323是一项“总括”建议，指的是各种ITU-T建议，尤其是建议H.225.0和H.245（基本通信能力）以及建议H。 450.1（支持补充服务的通用功能协议）。建议H.450.9规定了支持呼叫完成服务的H.323协议。 注:H.450.9仅适用于1998年版本的H.323（也称为H.323版本2）和更高版本。 在ISO/IEC 13870（QSIG）和ITU建议H.450.9（H.323）中，呼叫完成补充业务是完成对繁忙用户的呼叫（SS-CCB）和完成无应答呼叫（SS-CCNR）。由于被叫用户忙或不可用，这些补充服务在呼叫建立尝试失败后应用，并提供在被叫用户可用时重新建立呼叫的方法。 QSIG和H.323之间的互通允许在PISN用户处发起的呼叫在专用IP网络的用户处终止，或在专用IP网络的用户处发起的呼叫在PISN的用户处终止。在这种情况下，ISO/IEC 21991:2002允许被叫用户在忙（SS-CCBS）或未应答原始呼叫（SS-CCNR）后可用时完成呼叫。 采用QSIG的PISN和采用H.323的公共IP网络之间的互通不在ISO/IEC 21991:2002的范围内。然而，ISO/IEC 21991:2002中规定的功能原则上适用于与其他相关功能（例如:。 g、 、数字翻译、安全功能等）。 ISO/IEC 21991:2002适用于任何可以充当采用QSIG的PISN和采用H.323的专用IP网络之间网关的互通单元。

ISO/IEC 21991:2002 specifies signalling interworking between "QSIG" and "H.323" in support of call completion supplementary services within a Corporate telecommunication Network (CN). "QSIG" is a signalling protocol that operates at the Q reference point between Private Integrated services Network eXchanges (PINX) within a Private Integrated Services Network (PISN). The Q reference point is defined in ISO/IEC 11579-1. A PISN provides circuit-switched basic services and supplementary services to its users. QSIG is specified in other Standards, in particular ISO/IEC 11572 (call control in support of basic services), ISO/IEC 11582 (generic functional protocol for the support of supplementary services) and a number of standards specifying individual supplementary services. ISO/IEC 13870 specifies the QSIG protocol in support of call completion services. "H.323" is a set of signalling protocols for the support of voice or multimedia communication within a packet network, in particular a packet network that uses the Internet Protocol (IP) as its network layer protocol (IP network). H.323 signalling protocols operate between endpoints in an IP network, either indirectly via one or more gatekeepers, or directly. An endpoint can be a terminal or a gateway to another network. H.323 is an "umbrella" recommendation referring to various ITU-T recommendations, in particular Recommendations H.225.0 and H.245 (basic communication capabilities) and Recommendation H.450.1 (generic functional protocol for the support of supplementary services). Recommendation H.450.9 specifies the H.323 protocol in support of call completion services. NOTE H.450.9 applies only to the 1998 version of H.323 (also known as H.323 version 2) and to later versions. In both ISO/IEC 13870 (QSIG) and ITU-Recommendation H.450.9 (H.323), the call completion supplementary services are Completion of Calls to Busy Subscribers (SS-CCBS) and Completion of Calls on No Reply (SS-CCNR). These supplementary services apply after a call establishment attempt has failed because the called user was busy or not available, and provide means to re-establish the call when the called user becomes available. Interworking between QSIG and H.323 permits a call originating at a user of a PISN to terminate at a user of a private IP network, or a call originating at a user of a private IP network to terminate at a user of a PISN. In such a scenario, ISO/IEC 21991:2002 allows the completion of calls when the called user becomes available after having been busy (SS-CCBS), or having not answered the original call (SS-CCNR). Interworking between a PISN employing QSIG and a public IP network employing H.323 is outside the scope of ISO/IEC 21991:2002. However, the functionality specified in ISO/IEC 21991:2002 is in principle applicable to such a scenario when deployed in conjunction with other relevant functionality (e.g., number translation, security functions, etc.). ISO/IEC 21991:2002 is applicable to any interworking unit that can act as a gateway between a PISN employing QSIG and a private IP network employing H.323.