From what I've been able to tell, v15 has supported, and unsupported(aka modified) provisioning templates, even for supported devices.
"ProgramData\3CX\Instance1\Data\Http\Templates" is the location that official supported templates are stored and downloaded to.
"ProgramData\3CX\Instance1\Data\Http\Interface\provisioning\xxxxxxxxxx\CustomTemplates" is where it has been saving templates that I have made changes to.
My original solution was just to make changes to the official templates that reside in the first location that I mentioned above. At some point though, 3CX reset all the changes that I had made and set the templates back to default. I got around that by removing modify permissions from the templates that I modded for all except my service account.
You should be able to do the same. Add your custom address book info to the default template that is being used, and remove permissions for that file so that you are the only one that can edit it.
Personally I'm not a big fan of the way that templates are managed now. I get that 3CX wants to cover itself when things go poorly with custom templates. At the same time, there are very few people that will be satisfied with the setting in a default template. Compound that with the lack of an option to choose a template when assigning a phone, which really sucks. And the final cherry on top is the unsupported template warning...
Some changes need to be made to this process.
