The purpose of this European Telecommunication Standard (ETS) is to define a recommended set of supplementary services to the Teleservices and Bearer services which will be supported by a GSM PLMN in connection with other networks as a basis for the definition of the network capabilities required. The descriptions of the different supplementary services are contained in the GSM 02.80 series. Supplementary services not covered in GSM 02.04 or GSM 02.80 series cannot be introduced unilaterally in any GSM PLMN if they require modification of the GSM signalling Specifications. Technical realization of supplementary services is described in GSM 03.11 and 04.10.