This standard specifies the technical realisation of the first phase o f the network feature Support of Optimal Routeing (SOR). The first pha se of SOR provides: - a method to route a call from one mobile subscriber directly to anot her mobile subscriber who is in the same country as the calling mobile subscriber or in the called mobile subscriber?s home country, without needing to connect the call via the HPLMN of the called subscriber, e ven though the called mobile subscriber has roamed outside his HPLMN; - a method to forward calls when a called mobile subscriber who has ro amed outside his home country is busy, or is not reachable, or does no t reply, to a forwarded-to destination in the HPLMN country of the cal led subscriber or the VPLMN country of the called subscriber, without needing to connect the forwarded call via the VPLMN of the called subs criber; - a method to combine the optimal routeing described in the first bull et point above with the optimal routeing described in the second bulle t point above. OR of a call is permitted only if all entities involved in handling th e call support OR. Other cases of optimal routeing (e.g. calls where the calling and call ed subscribers are in different countries, forwarding to a mobile subs criber or multiple forwarding) will be considered for inclusion in lat er phases.