Hello, to get the message box back please follow these steps:
1. Make sure Zoiper is not running;
2. open a file browser (opening My Computer will do so);
3. type %APPDATA% in the address bar and press enter;
4. open "Config.xml" with notepad for editing;
5. search for error_message and change it to look like this:
<error_message>
<id>error_create_call</id>
</error_message>
Next time when you start Zoiper, the pop up message will be there again.