The President-elect dismisses his exit from diplomatic norms, but what impact will it have on relations with China?
It is difficult to overstate just how seriously China takes the issue of Taiwan.
As far as the government in Beijing is concerned, the island is an inviolable part of its sovereign territory, and its ultimate reunification is non-negotiable.
It has never renounced the possibility of using force to do so.
The issue has been the subject of decades of delicate diplomacy in Washington.
When Mao Zedong's Communists came to power in 1949, declaring the foundation of the modern People's Republic of China (PRC), the defeated Kuomintang, or Nationalist Party, under Chiang Kai-shek, fled across the water to Taiwan.
For the next two decades the United States continued to recognise his administration on Taiwan, referred to as the Republic of China, as the legitimate government of China, until the United Nations switched recognition to Beijing and the PRC in 1971.
President Jimmy Carter followed suit and shifted formal US relations to Beijing from Taipei in 1979.
The agreement included what became known as the "One China" policy, where the US government acknowledged Beijing's position, "that there is but one China, and Taiwan is part of China".
In 1992, Taipei and Beijing also agreed that there was only "one China" - agreeing to disagree on which was the legitimate government of it.
No US President, or President-elect has spoken to a Taiwanese leader since 1979, at least not that either side has publicly admitted, and despite unofficial ties and billions of dollars of "defensive" arms sales, Washington does not officially recognise Taiwan as an independent state.
Enter Donald Trump.
"The President of Taiwan CALLED ME today to wish me congratulations on winning the Presidency. Thank you!" the President-elect tweeted.
Batting away the storm this comment provoked, he continued, apparently unabashed: "Interesting how the US sells Taiwan billions of dollars of military equipment but I should not accept a congratulatory call."
China's foreign ministry has already
urging cautious and proper handling of the Taiwan issue, in order to avoid disturbance to Sino-US relations.
Privately, there will be real concerns as to where all this is heading.
One theory is that this is just Donald Trump being Donald Trump - bringing the freewheeling, throw-the-Washington-playbook-out-the-window style of his candidacy to his presidency.
His willingness to reject conventional wisdom, and the supposed liberal elite, was a powerful plank of his popular appeal after all.
But while that might work well on the campaign trail, it's hard to see how this scales up to the successful running of a country.
Earlier on Friday, for instance, Mr Trump also spoke to the Philippines president Rodrigo Duterte, who reported "a good rapport" and said the President-elect had wished him "success" with his anti-drugs campaign - this is the "campaign" in which more than 4,000 people have been shot dead, in months of extrajudicial killings.
The other, more worrying possibility for China, is that this was no spontaneous, enthusiastic blunder - that Donald Trump knows exactly what he is doing - and that he is indicating the start of a more significant shift in US-China, and US-Taiwan relations.
The Asia team he has assembled so far includes some pretty hawkish voices on China - from the economist Peter Navarro, author of "Death by China", to Michael Pillsbury, who's latest book purports to uncover "China's Secret Strategy to Replace America as the Global Superpower."
If this is Commander-in-Chief Donald Trump embarking on a strategic shift that is one thing, but at this level words and signals matter, and the President-elect's emerging approach to international diplomacy is unnerving more than just Chinese officials.
"It's Trump's right to shift policy, alliances, strategy," Chris Murphy, a democratic senator on the US senate foreign relations committee tweeted.
" What has happened in the last 48 hours is not a shift. These are major pivots in foreign policy without any plan. That's how wars start."
