The US has a very unique relationship with Taiwan. Although we don't officially recognize them, we still trade with them as if they were an independent nation (many American businesses are present in Taiwan and vice versa), sell them weapons, and have a defense treaty. The US has an embassy in Taiwan that serves every function of an embassy but is NOT called as such.
Given this information, you may think a phone call between the two nations isn't a big deal. It shouldn't be, but it is. The US president has not had any formal meeting with the Taiwanese president since 1979. I'm no expert of international arms deals, but I'm sure the US and Taiwanese government had a conversation between the two in order to make it happen. Just not so public. So nearly forty years of US foreign policy was just haphazardly thrown out the window with zero lead up at all. That's uh, not good. Fairly indicative of future foreign policy faux paus under the Trump administration. Diplomacy is a very complex art that is filled with imbalances and nonsense, but you need to know how to handle it effectively to keep people safe.
And my lord, why has no one STILL taken away Trump's twitter account? What purpose it is serving to his cabinet?