You assign them a power pack license, and then ensure is enabled in the user's User Settings in the Desktop Power User section.
You wouldn't be able to assign a power pack license if didn't have any unused licenses!
Also, in the Zoom app's General Settings, there is a section called Zoom Assistant. If the "Phone" checkbox is disabled, then the Power Pack will not be enabled regardless of the setting in the user's web portal settings. I believe this is because a similar feature to the power pack now exists for interacting with meetings.
Also, in the Zoom app's Settings, in the General section there is a section called 'Zoom Assistant' that interacts with the Power Pack as well. If disabled