I don't use software, but I use YouMail. It's an alternative voicemail service, but allows you to block calls, even give them a message to not call again if you like. It's quite good, and it's agreat alternative to my verizon voicemail.
www.youmail.com
raz
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Cum historia, Mutat valde Razgriz, Revelat ipsum,
Primum daemon scelestus est