According to the WSJ, T-Mobile was compromised in a significant Chinese telecom network hack.

The Wall Street Journal said on Friday, citing people familiar with the situation, that T-Mobile's network was one of the systems compromised in a destructive Chinese cyber-espionage operation that infiltrated several U.S. and foreign telecom firms. The Journal reported, without specifying when the attack occurred, that hackers associated with a Chinese intelligence agency had gained access to T-Mobile as part of a months-long effort to eavesdrop on the mobile conversations of high-value intelligence targets.

"T-Mobile is closely monitoring this industry-wide attack," a representative for the business emailed Reuters. "At this time, T-Mobile systems and data have not been impacted in any significant way, and we have no evidence of impacts to customer information." 

According to the WSJ investigation, it was unclear what information, if any, was obtained regarding the calls and communications records of T-Mobile subscribers. After breaching an unknown number of telecom businesses, China-affiliated hackers acquired surveillance material meant for U.S. law enforcement agencies, according to a statement released on Wednesday by the FBI and the U.S. cyber watchdog organization CISA.

The Journal revealed earlier in October that Chinese hackers gained access to the networks of American broadband providers, such as Lumen Technologies, AT&T, and Verizon Communications, and stole data from federal systems used for court-authorized surveillance.
Beijing has previously refuted allegations that it has employed hackers to access computer systems abroad made by the U.S. government and others.