The immediate occasion was an incident in which some hacker had gained access to a number of call records.

NBCnews headlined:

China targeted phones of Trump, Vance and Harris campaign affiliates, sources say
The FBI and Cybersecurity and Infrastructure Security Agency said the federal government is investigating.

A broad Chinese hacking campaign against U.S. telecommunication networks targeted the phones of Republican presidential nominee Donald Trump and his running mate, Sen. JD Vance, two sources familiar with the matter say. Another source told NBC News that people affiliated with the campaign of Vice President Kamala Harris were also targeted.

The staff of Senate Majority Leader Chuck Schumer, D-N.Y., was also targeted, a Democratic source said.
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In a joint statement, the FBI and the Cybersecurity and Infrastructure Security Agency said the federal government "is investigating the unauthorized access to commercial telecommunications infrastructure by actors affiliated with the People's Republic of China."
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The U.S. government recently concluded that China has hacked three American telecommunications companies: AT&T, Verizon, and Lumen Technologies. Lumen declined to comment. AT&T didn't respond to a request for comment.

Telecommunication companies keep metadata records of all calls made through their networks. The records include which number called what number at what time and from where. If one gets access to those records one can search for all calls a number associated with a person has made or received. But one (unless one is the NSA) does not get access to the content of those calls.