North Korea is cracking down on illegal cell phone calls with Russian made electronic devices according to Radio Free Asia (RFA).

The North Korean government has installed new radio locator devices along the country’s border with China. Cell phones smuggled into the country from China can pick up Chinese cell service near the border. North Koreans can use that cell service to make calls to outside the country, which is forbidden for virtually all North Koreans.

The new Russian devices allow the government to determine the locations of the illegal calls and monitor. The Russian devices replace older, weaker German models, according to RFA.

It is unclear how or where the country acquired the devices. North Korea is under strict economic sanctions imposed by the United Nations and other countries because of North Korea’s repeated ballistic missile and nuclear weapons tests. North Korea attempts to skirt the sanctions through smuggling and the black market whenever possible. The U.S. has also accused both China and Russia of not obeying sanctions.

“The State Security Department recently ordered installations of the newest version of additional radio locators along the border with China in Yanggang province,” a source who asked not to be named told RFA. “They are known to perform better than German radio locators that are already installed, so those who use illegal phone users are filled with terror about them.”

North Korea has its own legal cell phone market and an internal cell phone network. The regime uses people cell phones to monitor and track them. There were around 3 million network subscribers in the country as of 2015, according to Reuters.