scope:

This ETS specifies the supplementary service Call Retention which is applicable to various basic services supported by TETRA SwMIs. Basic services are specified in ETS 300 392-2 [4]. SS-CRT is a supplementary service which applies either during call establishment or once call is established to protect a call from pre-emption by another call of lower priority. Supplementary service specifications are produced in three stages, according to the method described in ITU-T Recommendation I.210 [2]. The present document contains the stage 2 specifications of SS-CRT. The stage 2 descriptions specify the supplementary services functional entities, their relations and the information flows needed to support the supplementary service.