This European Telecommunication Standard (ETS) gives the stage 3 description of the call transfer supplementary services. This ETS 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 call transfer 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 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 and coding for the supplementary services. Definitions and descriptions of supplementary services are given in GSM 02.04@ GSM 02.8x and GSM 02.9x-series. GSM 02.91 is related specifically to call transfer supplementary services. The technical realization of supplementary services is described in GSM 03.11@ GSM 03.8x and GSM 03.9x-series. GSM 03.91 is related specifically to call transfer 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. The following supplementary services belong to the call transfer supplementary services and are described in this ETS: - Explicit Call Transfer (ECT)