The present document specifies the stage 3 description of the Supplementary Service Include Call (SS-IC) for the Terrestrial Trunked Radio (TETRA). SS-IC permits the served user participating a call to include other participant to that call. Man-Machine Interface (MMI) and charging principles are outside 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 1 description specifies the service from the user's point of view (see ETS 300 392-10-17 [7]). The stage 2 description identifies the functional capabilities and the information flows needed to support the service as specified in its stage 1 description (see EN 300 392-11-17 [8]). The present stage 3 description specifies the protocols at the air interface and at the various Inter-System Interfaces (ISI) to support SS-IC. NOTE 1: According to ITU-T Recommendation I.130 [1]@ the stage 3 description of any telecommunication service addresses the network implementation aspects. Consequently it comprises two steps: the specifications of all protocols at the various reference points involved in any of the service procedures (notably the service operation) are the first step of the stage 3 description@ and the specifications of the functions of the corresponding network entities are its second step. NOTE 2: The latter have not been provided since they can be derived from the specification of the functional entity actions in the stage 2 description. The present document is applicable to Voice plus Data individual and group calls; more specifically to the following entities: - the MS/LSs of the served user and of the affected user; and - the served user and the affected user Switching and Management Infrastructures (SwMIs) in an individual and group call.