"Trump's specific language does not mark a break with U.S. policy — but the conversation itself does. China regards Taiwan — which broke away from Communist mainland China in 1949 — as an outlaw province. The U.S. severed diplomatic relations with Taipei in 1979 after President Jimmy Carter adopted a “One China” policy that recognized Beijing as China’s sole government.
No U.S. president or president-elect is known to have spoken to a Taiwanese leader since. But the U.S. does maintain friendly relations with Taiwan and -- as Trump noted -- has sold the island billions of dollars in military hardware, much to Beijing's ire."