This International Standard specifies the signalling protocol for the support of the Call Intrusion supplementary service (SS-CI) at the Q reference point between Private Integrated services Network exchanges (PINXs) connected together within a Private Integrated Services Network (PISN). SS-CI is a supplementary service which, on request from the calling user, enables the calling user to establish communication with a busy called user by breaking into an established call between the called user and a third user (unwanted user). The Q reference point is defined in ISO/IEC 11579-1. Service specifications are produced in three stages and according to the method specified in ETS 300 387. This International Standard contains the stage 3 specification for the Q reference point and satisfies the requirements identified by the stage 1 and stage 2 specifications in ISO/IEC 14845. The signalling protocol for SS-CI operates on top of the signalling protocol for basic circuit switched call control, as specified in ISO/IEC 11572, and uses certain aspects of the generic procedures for the control of supplementary services specified in ISO/IEC 11582. This International Standard also specifies additional signalling protocol requirements for the support of interactions at the Q reference point between SS-CI and other supplementary services and ANFs. NOTE 1 — Additional interactions that have no impact on the signalling protocol at the Q reference point can be found in the relevant stage 1 specifications. This International Standard is applicable to PINXs which can interconnect to form a PISN.