The present document for Mobile communications specifies the procedures used at the radio interface (reference point Um as defined in GSM 04.02) for normal operation@ registration@ erasure@ activation@ deactivation@ invocation and interrogation of community of interest supplementary services. The provision and withdrawal of supplementary services is an administrative matter between the mobile subscriber and the service provider and causes no signalling on the radio interface. In GSM 04.10@ the general aspects of the specification of supplementary services at the layer 3 radio interface are given. GSM 04.80 specifies the formats coding for the supplementary services. Definitions and descriptions of supplementary services are given in GSM 02.04 and GSM 02.8x and GSM 02.9x-series. GSM 02.85 is related to the community of interest supplementary services. Technical realization of supplementary services is described in technical specifications GSM 03.11 and GSM 03.8x and 03.9x-series. GSM 03.85 is related to the community of interest supplementary services. The procedures for Call Control@ Mobility Management and Radio Resource management at the layer 3 radio interface are defined in GSM 04.07 and GSM 04.08. Signalling interworking for supplementary services between GSM 09.02 and GSM 04.08 and between GSM 09.02 and GSM 04.80 is defined in GSM 09.11. The following supplementary services belong to the community of interest supplementary services and are described in the present document: - Closed User Group (CUG) clause 1.