Bearer and Teleservices, as respectively defined in GSM 02.02 and GSM 02.03, are the objects which the GSM PLMN operators offer to their customers. Besides these basic telecommunications services, features which aim at up-grading these basic services need also to be offered. Due to the use of radiocommunications in a PLMN, which are of a special nature compared to classical distribution transmission techniques used in the fixed networks, such a category of features is related to security aspects.