WASHINGTON, Sept 21 (Reuters) - A U.S. Justice Department-led committee is reviewing Zoom Video Communications Inc"s (ZM.O) proposed $15 billion all-stock deal to buy cloud-based call center operator Five9 Inc (FIVN.O), according to a letter filed with U.S. regulators. The Aug. 27 letter filed with the Federal Communications Commission said the Committee for the Assessment of Foreign Participation in the United States Telecommunications Services Sector -- known as "Team Telecom" -- is reviewing to see if the deal "poses a risk to the national security or law enforcement interests."