This is the response I received from the tech support at Nextiva...
Thank you for your email. Unfortunately, we are not able to set up the Polycom D60 at this time. We have made several unsuccessful attempts to set it up and are at this time waiting for our development team and Polycom to work out the bugs that are preventing us from using this device. Please let us know if you have any further questions.
I have been able to figure this one out. Provided your scenario only has one VoIP provider, enter the registration server and proxy settings at Settings > SIP. Make sure you get the dial plan and digit time out map from your VoIP provider. Then move over to Settings > Lines. Line 1 will be the first line that allows the base phone to register. Enter the SIP credentials, but moving forward, you will not enter any server settings for the lines. At this time, do not assign line 1 more than one line, you can adjust later, but not at this point. Lines 2 - 5 (depending on the number of VVX D60 handsets you have) will be for the VVX D60 handset registration, again you will enter the neccessary information from your VoIP provider along with SIP credentials. Again, as previously mentioned, we will not be entering server information on the lines. Lines 6 through what ever is left can now be used as additional line presences for Line 1.
After saving this, you can now set the line mappings for the VVX D60 handsets. You should be able to see from the handset status, that if you performed the above correctly, they are now registered.