North Korea has been restricting the use of SIM (Subscriber Identification Module) cards by foreign visitors. Tourists must buy a SIM card from the North Korean government when they arrive and surrender it when they leave. This is to make it more difficult for North Korean citizens getting foreign SIM cards that would enable them to access the worldwide Internet, and not just the local one (restricted to North Korea and boring as hell). In most parts of the world you can move your cell phone service from one phone to another by simply removing the small (25x14mm and smaller) SIM "card" from one phone and inserting it in another. SIM cards can also be bought just for the minutes stored on them. North Korea knows that SIM cards are easily smuggled in because they are so small. Similar sized cards (MicroSD) can carry up to 128 gigabytes of data and are a favorite for smuggling foreign (often South Korean) movies and TV shows. This really a losing battle, but the North Korean government sees control of information as a matter of life and death.
Increasingly since 2006 North Korea has been trying to block the use of anonymous (via untraceable SIMs) cell phones. The government has not been able to come up with a foolproof system that will prevent anyone from buying SIM cards that cannot be traced back to the purchaser. The North Koreans also know well that SIM cards are favored by gangsters and terrorists. North Korea has plenty of both on the payroll and outside the country these government thugs make heavy use of SIMs. Police have long noted that terrorist bombs are often set off using a cell phone with an anonymous SIM card. It's not uncommon to raid a terrorist hideout and find hundreds of anonymous SIM cards.
For years there has been a thriving black market in SIM cards. North Koreans with money (and a growing number of the new entrepreneurs do) can buy the SIMs and often cannot not run their businesses without these communications tools. While trying to control access to SIMs is futile, it is amusing to watch the North Koreans flail around at it.