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!
Try Root call blocker or Call master but both worked perfectly on versions like 2.3 or lower.From 4.1 and above work like all the others!
I hope i helped
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