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In the past, electronic eavesdropping against North Korea was carried out by an array of U.S. collectors, including offshore ships, aircraft and satellites. The chance of intercepting Mr. Kim’s personal phone calls remains remote.

Among the world’s electronic spying powers, the United State remains a leader. The U.S. government scored many intelligence coups for decades. One example was the 1970s program code-named “Gamma Guppy” that intercepted mobile telephone calls from Soviet leaders, including Communist Party Secretary Leonid Brezhnev.

North Korea has limited cellular communications technology: Its first mobile phones were introduced in the early 2000s and then banned in 2004. In 2008, Egypt’s Orascom began limited cellular service in the country.

By 2011, 1 million people used mobile phones in North Korea, according to Reuters, although calling in or out of the country is restricted, and Internet connections on the 3G network reportedly are blocked for most people.