Mexican billionaire Carlos Slim, who already controls America Movil, tightened his grip slightly on Latin Americas biggest phone company, according to a U.S. regulatory filing on Thursday that shows two companies controlled by Slim bought more shares. The purchases, made in November and December last year, come on top of a massive buyback effort by America Movil, whose share price has been under pressure since Mexicos government last year passed a reform plan that seeks to curb the companys dominance. Slims real estate company Inmobiliaria Carso and his bank Grupo Financiero Inbursa spent $212.5 million and $34.7 million purchasing 187.11 million and 30.98 million shares respectively in America Movil, a document sent to the U.S. Securities and Exchange Commission showed. Slim, his three sons, three daughters and their controlled companies and foundations now own about 47.6 percent of America Movils outstanding shares, up from 47.3 percent when the group made their last filing in November, according to a Reuters calculation.