ATHENS – It was kept secret but Greece’s telecoms operator Cosmote said the telephone data of thousands of customers was reportedly stolen by hackers, likely from Lithuania, from Sept. 1-5.

It was revealed now, the company said, so that it could conduct an investigation in secret about how it happened with no details about what kind of information may have been compromised on cell phones, such as banking.

The file concerns calls made through a cellular network and not a Global System for Mobile Communications (GSM) network, on which the calls of the direct messaging applications are based, said Kathimerini.

This file contained details of the calls made or received by mobile subscribers during those five days, specifically the phone number, the day and time of the call and its duration. Names were not included.

The least also included information on the device type, the International Mobile Subscriber Identity (IMSI), the age, gender, average revenue per user (ARPU), base station coordinates and mobile subscriber program, the paper reported.

“This data is used by the company to optimize the network and the services provided,” Cosmote said in a statement.