I have tried all the call blockers available on Android and i am sorry to say that no one is perfect!Many of them do almost what they promise but NONE of them blocks blacklisted numbers immediately.So as to mean that the blacklisted caller hears first the phone ringing and then hearing the busy tone.I personally find it really annoying!
Late, but maybe this information is useful for you,
If you have your phone rooted I recommend the best: ROOT CALL BLOCKER PRO, this is a paid version.
If your phone is not rooted I recommend BLOCKER (embware), it is free and is very similar to call blocker pro.
You will find both in Google play store.
I have tried both and for me are the best.
Regards,