A vulnerability in the SS7 inter-carrier network allows hackers and spies agencies to track locations and intercept all voice calls from anywhere in the world. SS7 allows cell phone carriers to collect location information from cell phone towers and share it with each other. This means a U.S. carrier will find its customer, no matter if he or she travels to any other country. There isn’t a fix yet, so the flaws in SS7 protocol will continue to be present even if your telephone carriers upgrade to more advanced technology.
Source: https://thehackernews.com/2015/08/hacking-phone.html