Umm.... why would you want to call them back? The best thing is to delete the calls. There is an app that lets you block calls from unknown numbers.
http://www.blackberryforums.com/bbf-...d-callers.html
There are some discussions on this app here, too.
Edit.. some other call blocking apps mentioned here:
http://www.blackberryforums.com/gene...-blocking.html