It is said

"President-elect Trump spoke with President Tsai Ing-wen of Taiwan, who offered her congratulations," the presidential transition team said on Friday in a readout of the phone call. "During the discussion, they noted the close economic, political, and security ties existing between Taiwan and the United States," it said. http://m.rediff.com/news/report/uspoll-trump-tears-up-diplomacy-phone-call-with-taiwan-could-irk-china/20161203.htm

what is the grammatical function of "it said" and actually what does it refe to?
please elaborate on its usag.