You also might want to check out YouMail, which is a free application and installs no software on your device.
Once you have it set up to your liking, you'll be able to do such fun things as custom greetings for those types of calls. You should hear some of the ones you can use. They're pretty funny. You can even set a "smart" greeting for those in your address book. "Hello RoGro. Clary33 is not available right now. Please leave a message".
Worth checking into at www.youmail.com.