Because one is a trade deal the other is a political signal of taiwan becoming recognized in a geographical area where china does not want to lose land, reason why political leaders also don't speak of an independent tibet.
The issued an official statement about it. So they are far from happy about it. Regardless of who called first Trump should have been informed about this, so this reflects poorly on him not being aware of it what means he wasn't informed and didn't think anything of it himself what is dumb, or he ignored the advice about it from his staff and those prepping him what is even worse.
Not surprised that people are still in the honeymoon period and don't fault him for anything, we seen this before with bush so not surprising in the slightest.
Also good you bring that twitter thing up, he played it down first and later there was an official statement from China about it. Do have to read all news about it in chronological order preferably
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Who called who first is not really the matter here. I explained this already so won't repeat that. The international press is reporting on it so either you have your tinfoil hat on and believe that they are all out to get Trump, a paranoia and anger he's succesfully building up against the press, what is another great trait as president being opposed to free speech, or China is far from happy about this and this will have consequences somewhere down the line, hence the official complaint from China urging the US to fall in line with the decades long plan of supporting an united china and not what they see legitimizing separatist states.
Anyway this wasn't the first phone call that raised eye brows, this will far from be the last.