This European Telecommunication Standard (ETS) specifies the procedure s used at the radio interface (reference point Um as defined in GSM 04 .02) for normal operation, registration, erasure, activation, deactiva tion, interrogation and network invocation of call offering supplement ary services. Provision and withdrawal of supplementary services is an administrative matter between the mobile subscriber and the service p rovider and cause no signalling on the radio interface. In GSM 04.10, the general aspects of the specification of supplementar y services at the layer 3 radio interface are given. GSM 04.80 specifies the formats and coding for the supplementary servi ces. Definitions and descriptions of supplementary services are given in GS M 02.04 and GSM 02.8x and 02.9x-series. GSM 02.82 is related specially to call offering supplementary services. Technical realization of supplementary services is described in GSM 03 .11 and GSM 03.8x and 03.9x-series. GSM 03.82 is related specially to call offering supplementary services. The procedures for Call Control, Mobility Management and Radio Resourc e management at the layer 3 radio interface are defined in technical s pecifications GSM 04.07 and GSM 04.08. The following supplementary services belong to the call offering suppl ementary services and are described in this ETS: - Call forwarding unconditional (CFU) (clause 1); - Call forwarding on mobile subscriber busy (CFB) (clause 2); - Call forwarding on no reply (CFNRy) (clause 3); - Call forwarding on mobile subscriber not reachable (CFNRc) (clause 4 ).