The current user mappings only assign a phone number based on the country code, area code, state, city, and postal code., not by site or any other attribute. When they add the unassigned user extensions to the account, they must assign a device and phone number in the region they are provisioning for, and they must assign the unassigned user extension to the site they will be using for that license.

It would be great if we had the option to provision users and assign them by 'Site.'

Add the 'Site' option to the Attribute Mapping