quite some time ago you said there are dynamic variables available in the new provisioning.
Like CFI_{EXTENSION} which will then result in CFI_11 CFI_12 and so on accordingly.
Is that the case? Because this is not working when i tested it: CFI_{EXTENSION}
This gets you the device name of the first account setup for the phone which if you didnt modify the device defaults will match the extension number,so you can use it for the keys you want.
$accounts[0]['device_name']
I will add, I use this variables directly in the custom section of the tenplates, have not tried them on the web form fields.