How long can Unicom call records be kept?

At present, China Unicom provides detailed list inquiry for mobile phone users in the past six months (including the current month), including call, SMS/MMS, value-added, data details and so on. You can log in to the mobile phone business hall or online business hall to print the detailed list, and the query interface of the mobile phone business hall or online business hall has printing options. I can also bring the owner's valid ID card and service password to the Unicom business hall to print the detailed list.