The issue might be caused by a corrupted / read only configuration folder or because Zoiper is not configured to save the contacts locally.
The config folder:
Zoiper 3 Biz keeps its config files in the Application Data\Zoiper folder for the windows user that started Zoiper. Usually this is the currently logged windows user.
Open a windows file explorer, type %APPDATA% in the address bar and press enter. Find the folder named Zoiper there and make sure the user that starts Zoiper has read/write permissions.
Your local configuration:
Run Zoiper, go to Settings -> Preferences -> Contacts -> Advanced and enable "Save local contact list".