I have a Netvanta 7100 on firmware R10.6.0. When I create a new user, in the phone model dropdown list, I do not see the VVX 3XX/4XX phone model listed. I understood that support for those phones was added in R10.6.0.
In IP Phone Globals, I do see those models listed in order to select the default firmware.
I would suggest loading AOS R10.8.0 as some changes were made for Polycom 300, 310, 400, and 410 VVX phones in that version. Just a note that only the ADTRAN branded Polycom VVX phones are supported. If that does not resolve the issue please respond and we can continue troubleshooting from there.