Every week, Mrs. Ri, a North Korea defector now living in Seoul, receives a call from her family back home. For her it is a chance to catch up with loved ones. For them, it is potentially a matter of life and death.
To make the calls, Mrs. Ri's relatives must run an elaborate gauntlet to call abroad on a Chinese mobile phone, smuggled in over the border. Fearing they could be shot if it is found in their home, they wrap the device in a plastic bag and bury it on a dusty hillside, digging it out to speak only at pre-arranged times.
"You mustn't talk more than five minutes. That's how they can trace you," said another North Korean, who defected in 2010.
Despite these risks, North Koreans are becoming increasingly connected to the world beyond their hermit kingdom, where most foreign media are banned and the Internet is virtually inaccessible.
Rapid growth in the use of mobile telephones is aiding the flow of information, and creating severe challenges for North Korea's rulers. Experts say it could gradually undermine both Kim Jong-un's police state and his propaganda machine.
Andrei Lankov, a professor of North Korean studies at Kookmin university in Seoul, said the state had to accept it could no longer monitor everyone, as it could in the days of only a few public phone boxes. "The authorities do their best to eavesdrop but they can hardly digest such a volume of traffic," he said.
For now, North Korean authorities are portraying the telephones as a boon to improve people's lives and business.
But the state has not always looked kindly on mobiles. Experts believe their introduction was blocked in 2004 after North Korean agents feared a mobile was used to trigger an explosion that may have been an attempt to assassinate the late leader Kim Jong-il.
Now that Pyongyang has softened its stance, the Nautilus Institute, a think-tank that does research on North Korea, argues the country will not be able to unwind this social and technological change.
North Korea will forsake "total control" and would shift to a model where "the government makes an example of a select group to try and force the rest of the country to stay in line, like the Chinese do," says Scott Bruce, a Nautilus director.
When Pyongyang's latest long-range rocket disintegrated shortly after blast-off 10 days ago, North Korea surprised observers with an unusual admission of failure, transmitted across the country on state television. In the past it has simply lied, telling the country that failed launches were successful.
Brian Myers, a North Korea expert at South Korea's Dongseo university, said Pyongyang had realised it could no longer lie so easily. "The only way to explain the admission is North Koreans' increased connectedness to outside sources," he said.
Access to outside information is still extremely limited in North Korea. Only very few officials and some local staff working at foreign embassies in Pyongyang have access to the Internet.
Some people have illegal shortwave radios, while businessmen regularly cross the border with China to trade in cities such as Dandong and Shenyang.
But it is the country's rapidly increasing mobile usage that is allowing news and gossip to spread with unprecedented speed.
North Korea has two types of mobile user. In the northern border areas, people use Chinese phones, which enable them to call abroad. Friends or fixers in China pay the bills.
Far more widely used are the phones operated by Egypt's Orascom, which cannot take international calls, but do aid the spread of information around the country. Orascom, which rolled out its network in 2008, says it has 1 million subscribers in the country of 24 million, soaring from 300,000 in 2010.
Mr. Lankov said it would be an exaggeration to say phones were already undermining state security but added that "the potential was there."
Traditionally, it has been hard to co-ordinate any protest from town to town because of a lack of communication links. Although mobile phones will change that, Yoo Dong-ryul, a security specialist at Seoul's Police Science Institute, said most users currently appeared to be bunched near to Pyongyang and the adjacent port of Nampo.
He added that prices of $200-$300 per phone put them mainly in the hands of officials and well-off traders.
Still, recent visitors to Pyongyang have expressed surprise at the number of seemingly ordinary workers in drab clothes who have a mobile phone.
Siegfried Hecker, a leading US expert on Pyongyang's nuclear program who has visited North Korea seven times, includes photographs of ordinary citizens chatting on mobiles in his presentations on the key factors determining Pyongyang's future.
"Time is not on their side," he said in a recent lecture. "Cell phones are going to get them in the end."