T-Mobile’s network was among the systems hacked in a damaging Chinese cyber-espionage operation that gained entry into multiple U.S. and international telecommunications companies, The Wall Street Journal reported on Friday citing people familiar with the matter.

Hackers linked to a Chinese intelligence agency were able to breach T-Mobile as part of a monthslong campaign to spy on the cellphone communications of high-value intelligence targets, the Journal added, without saying when the attack took place.

“T-Mobile is closely monitoring this industry-wide attack,” a company spokesperson told Reuters in an email.

“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.”

It was unclear what information, if any, was taken about T-Mobile customers’ calls and communications records, according to the WSJ report.

On Wednesday, The Federal Bureau of Investigation (FBI) and the U.S. cyber watchdog agency CISA said China-linked hackers have intercepted surveillance data intended for American law enforcement agencies after breaking into an unspecified number of telecom companies.

Earlier in October, the Journal reported that Chinese hackers accessed the networks of U.S. broadband providers, including Verizon Communications, AT&T and Lumen Technologies, and obtained information from systems the federal government uses for court-authorized wiretapping.

Beijing has previously denied claims by the U.S. government and others that it has used hackers to break into foreign computer systems.