In a recently declassified 1983 directive (pdf), President Reagan ordered steps to improve the security of government communications.
“Mobile and fixed communications systems used by key U.S. Government officials in the Nation’s capital and surrounding areas are especially vulnerable to intercept and exploitation by foreign intelligence services,” the President wrote.
National Security Decision Directive (NSDD) 113 was classified Top Secret until last year, when it was released in full. A copy was obtained by researcher Michael Ravnitzky. See “Security of Communications Systems Used by Key Government Officials,” NSDD-113, November 17, 1983.