WASHINGTON—A group of five U.S. senators urged the Federal Communications Commission (FCC) on Wednesday to address surveillance threats posed by foreign firms providing services to U.S. telecommunications providers. The previously unreported letter, led by Sen. Ron Wyden (D-Ore.) and also signed by Sens. Dick Durbin (D-Ill.), Richard Blumenthal (D-Conn.), Chris Van Hollen (D-Md.), and Ed…