NOT SURE IF IT WORKS WITH WM6.5, HOWEVER
MagiCall is a smart, rule based filter software for SMS filtering and dual-way calls filtering.
It gives you the ability to
block unwanted callers with SMS replies.
It can also add prefixes automatically in front of the original called numbers or hang them up depending on your settings.
In addition, you can take control of incoming SMS and more!
Not Free, Find it here.... Or PM me for other details!!!:P Hope this Helps!!!
Note: i personally like this software, as you can block calls, but not only that, set it up to simply pickup and hangup so not to allow the ability to leave a message!