Microsoft has won US antitrust approval to buy the Internet phone service Skype, the Federal Trade Commission said in a website posting on Friday. Microsoft announced in May it was buying Skype for $8.5 billion, its biggest-ever acquisition, placing a rich bet on mobile and the Internet to try and best rivals such as Google Inc.The approval was announced in a listing of deal approvals that comes out several times a week. Microsoft's interest in the money-losing, but popular service highlights a need to gain new customers for its Windows and Office software. Skype has 145 million users on average each month and has gained favor among small businesses.