https://arstechnica.com/information-technology/2021/10/company-that-routes-sms-for-all-major-us-carriers-was-hacked-for-five-years/

Syniverse, a company that routes hundreds of billions of text messages every year for hundreds of carriers including Verizon, T-Mobile, and AT&T, revealed to government regulators that a hacker gained unauthorized access to its databases for five years. Syniverse and carriers have not said whether the hacker had access to customers' text messages.

A filing with the Securities and Exchange Commission last week said that "in May 2021, Syniverse became aware of unauthorized access to its operational and information technology systems by an unknown individual or organization. Promptly upon Syniverse's detection of the unauthorized access, Syniverse launched an internal investigation, notified law enforcement, commenced remedial actions and engaged the services of specialized legal counsel and other incident response professionals."