Lync Phone automatically sign-out

Hi,

I configure a "normal user" (not common area phone) with an IP Phone (HP 4120). If I unplug the USB and network cable (after the configuration) between the phone and the PC (the ethernet cable still pluged in of course), after 5 minutes the user will automatically sign-out and the phone give an error message, that the user is not authorized to sign-in. I don't know what is the reason of this problem. Can you help me about that?

Many thanks,

Tamás