As per following reference here and here this is general error that means the phone cannot authenticate with the SIP proxy. Please check the link above and check if the resolution there able to help you

My Poly CCX 600 is doing the same. Service is interrupted each time it does this. I have been on tech support call with Google Fiber, Poly and Ring Central and no one seems to be able to fix this. This is a remote office in NC amd my firm’s IT guy is 4k miles away in Alaska.

The Line:Unregistered error message is usually accompanied by a pop up box that says 

VLAN ID:Native followed by the phones IP address

I will add that with the help of Ring Central support have reset the phone to factory default, reprovisioned phone and updated software all to no avail. 

It has done this occasionally over the past two years but not enough to cause major issues. However, two days ago I switched from ATT Fiber to Google Fiber and now this is happening at least once every minute or two rendering the phone useless.