America Movil, controlled by billionaire Carlos Slim, owned nearly 30 per cent of KPN until the foundation exercised an option to give itself about 50 per cent of voting stock.
Shares in America Movil rose more than six per cent following the news.
“America Movil believes, and so told representatives of KPN, that KPN’s shareholders should be the ones to decide whether the price offered is fair,” the firm said in a statement yesterday.
“They should have had the option to sell their shares, considering the terms and conditions of the intended offer,” it added.
Slim’s holding had been losing money when he backed KPN’s plan to slim down earlier this year.
KPN raised €3bn in a heavily-discounted rights issue in April and agreed to sell its German unit to Telefonica for €8.5bn last month.