ISO/IEC 15991:2003规定了补充业务呼叫优先级中断（SS-CPI）和呼叫优先级中断保护（SS-CPIP），适用于专用综合业务网（PISN）支持的各种基本业务。ISO/IEC 11574规定了基本服务。 注:本版ISO/IEC 15991不适用于电路模式多速率承载业务。 SS-CPI是一种补充业务，在没有可用的用户信息信道的情况下，允许优先呼叫的呼叫请求成功进行。这是通过强制释放低优先级的既定呼叫来实现的。 SS-CPIP是一种补充业务，允许保护呼叫免受其他优先级呼叫的干扰。 SS-CPI和SS-CPIP是分开描述的，因为SS-CPI是主叫用户使用的服务，而SS-CPIP是主叫用户或被叫用户使用的服务。 根据ETS 300 387中描述的方法，补充业务规范分三个阶段生成。ISO/IEC 15991:2003包含SS-CPI的第1阶段和第2阶段规范。第1阶段规范（第6条）规定了一般特征原则和能力。第2阶段规范（第7条）确定了补充服务中涉及的功能实体以及它们之间的信息流。

ISO/IEC 15991:2003 specifies the Supplementary Services Call Priority Interruption (SS-CPI) and Call Priority Interruption Protection (SS-CPIP), which are applicable to various basic services supported by Private Integrated Services Networks (PISN). Basic services are specified in ISO/IEC 11574. NOTE This edition of ISO/IEC 15991 does not apply to the circuit-mode Multiple Rate Bearer Service. SS-CPI is a supplementary service that allows a call request for a priority call to proceed successfully in the case that there is no user information channel available. This is accomplished by force releasing an established call of lower priority. SS-CPIP is a supplementary service that allows for the protection of calls against interruption from other, priority, calls. SS-CPI and SS-CPIP are described separately because SS-CPI is a service used by a calling user, and SS-CPIP is a service used by either a calling user or a called user. Supplementary service specifications are produced in three stages, according to the method described in ETS 300 387. ISO/IEC 15991:2003 contains the stage 1 and stage 2 specifications of SS-CPI. The stage 1 specification (clause 6) specifies the general feature principles and capabilities. The stage 2 specification (clause 7) identifies the Functional Entities involved in the supplementary service and the information flows between them.