Hello,
You would need to open Finder, then hit "CMD + SHIFT + G" keys on your keyboard.
On the shown pop-up type ~/Library/Zoipier5, confirm.
Locate the "ContactsV2.db" and "HistoryV2.db" files, make sure Zoiper is not running on background and delete those two files.
Note, this will remove both the contacts and the history, however the history database is linked with the contacts and should be also cleared with them.
