Exporting of contacts is not possible.
However, they are saved in "~/Library/Zoiper/Contacts.xml".
In Zoiper5/Windows the new place is:
it is in SQLite-Format, a free database. There is a SQLite-Browser for windows in https://sqlitebrowser.org
On Mac the location of the contacts data is inside this sqlite database:/Users/christian/Library/Zoiper5/ContactsV2.dbThis can be opened in sqlitebrowser. The tables Name and Phone which contain the most important data can be exported with sqlitebrowser as csv files and combined using the Contact ID. But it would take some scripting to transform this into a usable phone list.
This is for Zoiper5, while PatrickPinetco was asking about Z3.
Zoiper3 saves the contacts in plain XML which makes it easier to be exported with the help of some online XML to CSV convertor. I have tried it myself and generally works, but it is not an officially supported or recommended method. : )