“Nothing can ever justify people being thrown in detention for trying to fulfill a basic human need — to connect with their family and friends,” said Arnold Fang, the author of the report, which relied on interviews with experts and 17 recent defectors from North Korea.

Mr. Kim’s clampdown on phones linked to Chinese mobile networks also heightens the risk for those who help bring news about his totalitarian country to the outside world. North Koreans use the phones to talk or send text messages and even photos to reporters and activists in South Korea and elsewhere.

If they are caught by officials from the North, bribes are virtually the only way to avoid prison, or worse.

“When my sources call me, they shut the doors of the house and keep a lookout outside,” said Kang Mi-jin, a reporter for Daily NK, a news website based in Seoul that focuses on the North. “I tell them to have a place to quickly hide their phone and carry bribe money, usually 2,000 Chinese yuan, with them, always. It can decide whether they live or die.”

Thanks to her sources in North Korea, Ms. Kang, 48, broke some of the most talked-about news on Mr. Kim’s secretive government in recent years. She was the first to report that Mr. Kim’s wife, Ri Sol-ju, was pregnant in 2012 and that the leader was limping in 2014 because of ankle surgery.

“The people I talked to in the North are thirsty for outside news — asking as many questions of me as I do of them,” said Ms. Kang, herself a defector. “They want to know how defectors live in the South, how much a South Korean worker makes a month, whether it’s really true that South Korean housewives have so many pieces of clothes they throw some away.”

Image Ju Chan-yang, a North Korean defector who came to South Korea in 2011, at Korea University, in Seoul. Credit... Jean Chung for The New York Times

North Korea runs its own mobile phone network. Started in 2008 as a joint venture with the Egyptian company Orascom, the network, Koryolink, has more than three million subscribers. But it does not allow international calls. For ordinary citizens, landline calls are monitored and mostly confined to domestic connections.