MEXICO CITY, Dec. 13 -- The ongoing dispute between Telmex, AT&T and WorldCom may have to be settled by the World Trade Organization. AT&T and WorldCom claim that Telmex, Mexico's largest wireless firm, still has not followed through on its promise to open up Mexico's $12-billion telecommunications market after four years of competition. Telmex still controls about two-thirds of the long-distance market and nearly all local service, according to a report in the Wall Street Journal.