At first, the F.B.I. and other investigators believed that China’s hackers used stolen passwords to focus mostly on the system that taps telephone conversations and texts under court orders. It is administered by a number of the nation’s telecommunications firms, including the three largest — Verizon, AT&T and T-Mobile. But in recent days, investigators have discovered how deeply China’s hackers had moved throughout the country by exploiting aging equipment and seams in the networks connecting disparate systems.

https://www.nytimes.com/2024/11/21/us/politics/china-hacking-telecommunications.html

Doesn’t look like they know (or are willing to share specifics as to) the full scope of the hack, but they seem to have a pretty good idea.