When a Lync phone (in my scenario a Polycom CX700 and a Polycom CX3000) is connected to a Computer using a USB cable you can use the Lync client on the PC to logon to the Lync servers. An additional pop-up is shown where the credentials have to be entered:
But the Lync client will not logon and the pop-up appears all the time. At the same time you may see a message like “Cannot download certificate because domain is no accessible. If the problem persists, contact your system administrator” or something like this.