The present document defines the group call communication of interworking at the Inter-System Interface (ISI) for Terrestrial Trunked Radio system (TETRA) supporting Voice plus Data (V+D). 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: - Transport Layer Independent Specification ISI general design [2]; - General Design, PSS1 over E.1 [i.7]; - General Design, SIP/IP [i.8]; - Transport Layer Independent Additional Network Feature - ISI Individual Call (ANF-ISIIC) [3]; - Transport Layer Independent Additional Network Feature Group Call (ANF-ISIGC) (the present document); - Transport Layer Independent Additional Network Feature Short Data service (ANF-ISISD) [i.9]; - Transport Layer Independent Additional Network Feature Mobility Management (ANF-ISIMM) [4]; - Generic Speech Format Implementation [i.10].