No sure if this is the correct place to report false positives, but Zoiper Free (a voip software package) has the following report:

C:\Program Files (x86)\Attractel\Zoiper\Zoiper.exe: Trojan.Packed-142 FOUND

ClamWin then, correctly, shuts down the application. But, according to AVG and other virus scanners Zoiper is clean software.

What should I do?

You should report false positives to Clam Antivirus (which provides the scan engine/database) for ClamWin, starting at https://www.clamav.net/sendvirus/ on the web. When you get to the page that lets you upload the file involved, please be sure to check the false positive block, give them the exact name of the falsely detected virus, and tell why you thing it is a false positive in the comments section. In your case, mention the lack of detection by other scanners.

Regards,