So the provisioning address should simply be the public https://IP/FQDN address of the PBX/xepm-provision/ ? and then in endpoint manager the mac should be added and this should allow the device to obtain configuration credentials automatically, correct?
Maybe you check out the new Provisioning as well. Its a new addon.
Link for auto-provisioning can be copied with push of a button.
Phones will ad the [mac-number].cfg automatically, too. At least Fanivl does it.