Sometimes i recive:
SIP 500 - Server Internal Error
This issue is not caused by Zoiper. It is only showing the error that is sent by the server. Please report the issue to your provider.
i get this error alot and would really like to know more about what is happening. oftentimes i'll just keep getting this error until i reboot the phone, then it works. so it doesn't seem likely to be an issue with my provider.