Make sure you got SSL(Letsencrypt) enabled and go into the phone set the provision URL to use https and put in your URL <yourFQDNhere>/app/provision/ if needed can automate this with dhcp option 66 for larger deployments.
At one point you could provision Polycom with http but after a certain version they made it only work with https. I had to open a ticket with the developer to learn this would only work with SSL to provision.
I use the VVX411 and VVX601 phones
