The fun part is they don't know the extent of the comprise or how long it has been going on.
What happened is that CISA recently published a report stating that they think a lot of US telecommunications equipment has been compromised. It isn't a one time breach. They know that China has control over a unspecified amount of critical components. The malware China is using is extremely complex and very hard if not completely impossible to detect. China is very good at covering there tracks so even getting a sample of Malware is hard.
Because of all this, CISA is now recommending that people use encrypted messagers.
My understanding is that the scope is totally unknown. I'm sure they exploited the crap out of those 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.