Base on the new , cloud pbx should support version version 4.0.7577.4463.
We have upgrade to this version and test it can't sign in throught USB. But USB is only one method to login.
Credential like below screen.
cx600 screen pending on connecting to Lync Server...
Polycom and other manufacturers produce these optimized for Microsoft® Lync® devices after a reference design and the embedded OS running on the phones is provided, supported and maintained by our partner Microsoft®.
You therefore may want to reach out to the Microsoft® community and / or Microsoft® support directly instead as this is the most direct and efficient path.
Please try here