SEOUL >> The man drove Ju Chan Yang to a mountain overlooking North Korea’s border with China. He looked around, making sure they were alone. Then he dialed his Chinese cellphone and handed it to her.

On the other end was Ju’s father in South Korea. It was the first time they had heard each other’s voice since her father fled there two years earlier.

“We barely spoke 10 minutes before the connection was suddenly lost,” Ju, 25, said, describing the 2009 episode during a recent interview. “My father lost sleep that night, fearing that I might have been caught by North Korean soldiers.”

Smuggled-in Chinese mobile phones, which enable North Koreans near the border with China to gain access to its mobile networks, are an increasingly vital bridge between the North and the outside world.

They connect North Koreans to relatives who have defected abroad, mostly to South Korea. North Koreans also flee their repressive country, as Ju eventually did, through a smuggling operation arranged by mobile phones.

But the bridge has become increasingly precarious under Kim Jong Un, the North Korean leader.

In 2014, Kim ordered his government to tighten “mosquito nets” to block foreign information from slipping in and prevent his people from communicating with outsiders and fleeing. The number of North Korean defectors arriving in South Korea, once as high as 2,914 in 2009, plummeted to 1,276 last year.

Kim also strengthened a crackdown on mobile phones smuggled from China, deploying more soldiers and modern surveillance devices along the border to jam signals or trace them to those using the banned phones.

In a 57-page report titled “Connection Denied,” Amnesty International said this month that North Koreans caught making calls on the phones could face criminal charges. If they call someone in South Korea or other countries labeled enemies, they could face charges of treason as well as incarceration in prison camps.

“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.

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, Kang, 48, broke some of the most talked-about news on Kim’s secretive government in recent years. She was the first to report that 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 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.”

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 3 million subscribers. But it does not allow international calls. For ordinary citizens, landline calls are monitored and mostly confined to domestic connections.

Internet access is also restricted to foreign visitors and a select elite. North Koreans are also not permitted to exchange letters, emails or telephone calls with people in South Korea.

Thus, for ordinary North Koreans, virtually the only means of communicating directly with outsiders is to travel to the border with China and use cellphones that have been smuggled in.

North Korean traders began using cellphones during a famine in the 1990s to help illegally bring food and other goods from China.

But North Koreans who have fled to the South since the famine also started hiring smugglers to send Chinese phones and SIM cards to relatives left behind. Defectors and Amnesty International said the illicit trade in Chinese mobile phones in the North was growing.

In 2008, Choi Hyun Joon, 51, a North Korean defector in Seoul, hired a middleman in China. The middleman called his contact in North Korea on his Chinese cellphone and told him to find Choi’s daughter, Choi Ji Woo, in Pyongyang, the North Korean capital.

It took two months for the middleman to get his daughter to the border for a call with Choi. He first had to forge a travel permit for her; in North Korea, travel from town to town is closely monitored.

Later, Choi wired his daughter 8 million won, or $6,700, in a transaction made possible by the Chinese mobile phone. Each year, defectors in South Korea send millions of dollars to their families in the North through intermediaries in China and North Korea who use Chinese mobile phones to arrange the transactions.

By the time the money reached her, the sum had shrunk by half, the rest deducted as “brokers’ fees” for the middlemen.

“You lose 30 to 50 percent of the money, but still it is the only way to send money to our loved ones,” said Choi, who managed to get his daughter out in 2010.

Ju remembered the day that the middleman her father had hired appeared at a workers’ restaurant in Chongjin, a port city in northeast North Korea, where she was working in 2009.

He discreetly showed Ju a code word only she and her parents knew, a combination of her and her mother’s names.

“I knew I could trust him,” Ju said. “He said I should go to the border to make a phone call with my parents in the South.”

In the summer of 2010, her father called her to the Chinese border for the last time. There, she was handed over to a North Korean guard who was part of a smuggling operation. He helped her swim across a rain-swollen river to China. Once there, he used his cellphone to call another middleman to come pick her up — and pay his fee.

“The soldier had his Chinese cellphone wrapped in plastic and between his teeth when he swam across the river,” said Ju, who arrived in South Korea in 2011. “The phone is what links North Korea to the outside world.”

© 2016 The New York Times Company