This ETS specifies the procedures used at the radio interface (reference point Um as defined in technical specification GSM 04.02) for normal operation, registration, erasure, activation, deactivation, interrogation and network invocation of call offering supplementary services. Provision and withdrawal of supplementary services is an administrative matter between the mobile subscriber and the service provider and cause no signalling on the radio interface. In technical specification GSM 04.10 the general aspects of the specification of supplementary services at the layer 3 radio interface are given. Technical specification GSM 04.80 specifies the formats and coding for the supplementary services. Definitions and descriptions of supplementary services are given in technical specifications GSM 02.04 and GSM 02.8x and 02.9x-series. Technical specification GSM 02.82 is related specially to call offering supplementary services. Technical realization of supplementary services is described in technical specifications GSM 03.11 and GSM 03.8x and 03.9x-series. Technical specification GSM 03.82 is related specially to call offering supplementary services. The procedures for Call Control, Mobility Management and Radio Resource management at the layer 3 radio interface are defined in technical specifications GSM 04.07 and GSM 04.08. The following supplementary services belong to the call offering supplementary 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).