This International Standard specifies the Call Intrusion supplementary service (SS-CI), which is applicable to various basic services supported by Private Integrated Services Networks (PISN). Basic services are specified in ISO/IEC 11574. Call Intrusion (SS-CI) is a supplementary service which, on request from the served user, enables the served user to establish communication with a busy called user (user B) by breaking into an established call between user B and a third user (user C). Supplementary service specifications are produced in three stages, according to the method described in CCITT Recommendation 1.130. This International Standard contains the stage 1 and stage 2 specifications of SS-CI. The stage 1 specification (clause 6) specifies the supplementary service as seen by users of PISNs. The stage 2 specification (clause 7) identifies the functional entities involved in the supplementary service and the information flows between them. Note — For this International Standard, stage 2 does not consider the split of functionality between a functional terminal at user B and the local PINX. Terminal functions and local PINX functions at user B are included in the same Functional Entity.