This Supplementary Service describes the procedures and the signalling protocol for the Call Intrusion supplementary service in H.323 (Packet Based Multimedia Communications Systems) networks. The Call Intrusion supplementary service (SS-CI) enables a calling user A, encountering a busy destination user B, to establish communication with user B by breaking into an established call between user B and a third user C. This Recommendation makes use of the Generic functional protocol for the support of supplementary services in H.323 as defined in ITU-T H.450.1. This Recommendation requires H.323 version 2(1998) or later. Version 2 products can be identified by H.225.0 messages containing a protocolIdentifier= {itu-t (0) recommendation (0) h (8) 2250 version (0) 2} and H.245 messages containing a protocolIdentifier= {itu-t (0) recommendation (0) h (8) 245 version (0) x}, where x is 3 or higher. The procedures and the signalling protocol of this Recommendation are derived from the Call Intrusion supplementary service specified in ISO/IEC 14845 and ISO/IEC 14846.