The Logan Act makes it illegal for any American to communicate with a foreign government in relation to a position contrary to the US Administration.
Donald Trump is not even President-elect yet. The electoral college does not vote until December 19th and electors are actively changing allegiance day-by-day.
No US president has spoken directly to Taiwan's leader in 37-years because of the damage it would do to our relationship with China, the world's second most powerful nation. Further, Trump referred to the president of Taiwan as the representative of the "government of Taiwan" contrary to longstanding policy.
This act is damaging to national security and Trump's own position when re-negotiating trade with China, a key plank in his own campaign.