The present document specifies the stage 2 description of the Supplementary Service Include Call (SS-IC) for the Terrestrial Trunked Radio (TETRA). SS-IC enables a served user@ while already participating in an active call (original call) to include new participant(s) into a group call together with the participant(s) in the original call. Man-Machine Interface (MMI) and charging principles are outside of the scope of the present document. Supplementary service specifications are produced in three stages according to the method defined in ITU-T Recommendation I.130 [1]. The stage 2 description identifies the functional capabilities and the information flows needed to support the supplementary service as specified in its stage 1 description (see ETS 300 392-10-17 [7]). The stage 2 description is followed by the stage 3 description@ which specifies the protocols at the air interface and at the various Inter-System Interfaces (ISI) to support the service. The present document is applicable to TETRA Voice plus Data terminal equipment and networks.