I had the same problem with VoIP Buster. It worked fine in Windows Vista, but gave this error after I updated to Windows 7.
This is what worked for me:
- check for updates of the sound drivers on your computer or laptop. In my case, 2 sound drivers, where present, I checked for updates and one of them needed an updated. After the update VoIP buster worked fine again!

This solution was sugested to me by the VOip helpdesk.