I can't believe I'm defending D. Trump, but here it goes:
Trump's done some stupid things during the election, but this isn't a dumb move. The current relationship that China has with America if simplified, would be like this: China reaps massive economic benefits from trade with the US, while some American companies (almost always large-sized ones) benefit from cheap Chinese manufacturing, and sometimes access to the Chinese market as well. The winners: China, and some US companies. The losers: everyone else in the US.
Trump doesn't (yet) have ties with a lot of the US companies operating in China, so it doesn't put him or the US in general in a bad position if he wants to renegotiate trade relations with China, and in the process get popularity at home from appearing to support domestic manufacturing. The only thing he did by receiving that phone call was gain leverage for negotiations. It's a good move, and I guarantee you China was planning to do something similar if they didn't already.