A system to be used in fixed and mobile telephone networks, based on telephone set types (1), (3) and (8) that have the capacity of storing in a memory (21) a number of carrier identification codes organized in table form (36) and of performing the automatic selection of the least cost carrier to route a call initiated by the user. Such telephone sets also capable of connecting periodically and automatically to an information provider (6) that contains a users database (9) and a tariffs database (10), both permanently updated, for the purpose of keeping updated the information stored in memory (21). All the processing to determinate the least cost route is performed at the information provider (6) and information is sent to the user's telephone set (1), (3) and (8) on demand.