The present document defines the Terrestrial Trunked Radio system (TETRA) supporting Voice plus Data (V+D). It specifies: - general design aspects (e.g. reference points, numbering and addressing, or protocol architecture); - the interworking between TETRA networks; - the interworking of TETRA networks with other networks, via gateways; - the supplementary services applicable to the basic TETRA tele- or bearer services. The TETRA V+D interworking - basic operation part defines the interworking between TETRA networks over the corresponding interface: the Inter-System Interface (ISI). It comprises the following sub-parts: - ISI general design; - Additional Network Feature - ISI Individual Call (ANF-ISIIC); - Additional Network Feature - ISI Group Call (ANF-ISIGC); - Additional Network Feature - ISI Short Data Service (ANF-ISISD); - Additional Network Feature - ISI Mobility Management (ANF-ISIMM); - Speech Format Implementation for Circuit Mode Transmission; - Speech Format Implementation for Packet Mode Transmission. The present document is the ANF-ISIGC sub-part.