https://www.vice.com/en_us/article/z3xpm8/company-that-routes-billions-of-text-messages-quietly-says-it-was-hacked

A company that is a critical part of the global telecommunications infrastructure used by AT&T, T-Mobile, Verizon and several others around the world such as Vodafone and China Mobile, quietly disclosed that hackers were inside its systems for years, impacting more than 200 of its clients and potentially millions of cellphone users worldwide. 

The company, Syniverse, revealed in a filing dated September 27 with the U.S. Security and Exchange Commission that an unknown "individual or organization gained unauthorized access to databases within its network on several occasions, and that login information allowing access to or from its Electronic Data Transfer (EDT) environment was compromised for approximately 235 of its customers."