Hello
from the quoted FAQ:
Since UC Software 5.8.0 the REST API is publicly available to everybody.
The VVX 411 may have unofficially supported this prior to 5.8.x but officially and usable and supported is the feature only since 5.8.0
Upgrade your phones to a currently supported Version like UC Software 6.1.0, UC Software 6.0.0 or UC Software 5.9.3
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Best Regards
Steffen Baier