Zoiper3 for Linux keeps the contacts in a file named Contacts.xml
You can remove this file by following these steps:
A: Using a graphic file explorer application:
1. make sure zoiper is not running;
2. open your file explorer application such as Nautilus, Thunar, etc.;
3. Select Go -> Open location;
4. type ~/.Zoiper in the address bar;
5. confirm your choice;
6. Rename the file named Contacts.xml;
B: Using a terminal emulator:
1. make sure zoiper is not running;
2. open a terminal emulator like konsole, xterm, uxterm, etc.;
3. type the following command:
cd ~/.Zoiper
4. press Enter;
5. Rename the file named Contacts.xml;
mv Contacts.xml Contacts.xml.old
6. press Enter;
Next time you start Zoiper there will be no contacts.