|This? This means nothing|
The American governmental leadership has agreed to participate in this farce since 1972, and in 1979 President Carter withdrew diplomatic recognition from Taipei. No high level contacts between the US and Taiwan have occurred since. In lieu of an embassy, most western nations (who also play along with the One China policy) have private "Trade Missions" that work with the Taiwanese government - and much business is done. The US is the primary supplier of weapons to the ROC, and for decades America has maintained a weird passive-aggressive 'ambiguity' about when and under what circumstances we might intervene in a cross-strait war.
This is all - quite obviously - transparent farce. The two key underpinnings of American (and Western) acceptance of the One China policy are a.) Respect for Beijing and the PRC government's diplomatic sensibilities in not supporting Taiwanese independence and b.) maintenance of an American 'strategic ambiguity' in the China sea. Those two approaches do not require the precise formula the US has employed over the last forty years. One could easily imagine a One China policy that included high level political contacts. The government in Beijing would complain, but as an adversary power they tend to complain about a lot of US actions - just as we complain about many Chinese policies. They complain when we have contact with the Dalai Llama, fer crissakes.
Whether or not Trump was aware of the historical diplomatic issues in play when he spoke to President Tsai, let's be careful not to overreact to the call. It won't lead to war - it won't even necessarily lead to a change in US policy. Of course, THAT'S not entirely certain. In the narrow, through-the-looking-glass Trumpian world view, China (along with Mexico) are the global villians that have out-smarted, out-played and out-negotiated the US for decades. If he were to decide to actually act on this demented worldview (and make no mistake, it would be in spite of the resistance of most of his more rational advisers), then it becomes somewhat difficult to predict the form of those policy changes. Tariffs on goods are possible, but if he wanted to play hardball without starting a trade war perhaps shifting US policy toward Taiwan might be considered.
Lots of things are possible, and most of them are bad. But right now this is a nothingburger.