This Stage 1 is a description of the first phase of Support of Optimal Routing (SOR): - compiles the basic service requirements for SOR; - describes the interactions for Supplementary Services (SS) in order to cater for SOR; - refers to modifications to network features required by SOR. This Global System for Mobile communications Technical Specification ( GTS) does not address the following: - There is no need for optimization of the routing of calls originally directed to a fixed network subscriber, because the physical address of a fixed network terminating line cannot differ from its logical add ress. - SOR in non-GSM mobile networks is not a subject of this GTS, but mig ht be possible by bilateral arrangement between GSM network operators and those non-GSM mobile network operators. The purpose of SOR is to reduce the number of unnecessary inter-PLMN c all legs. The first phase of SOR applies to: - OR performed within a country (i.e. Mobile to mobile calls where bot h mobile subscribers are located in the same country); - OR performed towards the country where the call would have normally been routed without OR (e.g. Conditional Call Forwarding towards the H ome Country of the called party). The complete set of scenarios included in the phase 1 of OR is present ed in the paragraph for normal procedures. All other scenarios are exc luded from Phase 1 of OR. Note that Optimal Routing is applicable to national roaming situations , that is to calls directed to a mobile subscriber roaming in her home country, but registered in a PLMN different from her HPLMN. All further call scenarios, including multiple call forwarding, are le ft to subsequent phases of SOR. Subsequent phases of SOR shall be back wards compatible with this first phasd.