If you block a phone number, how is the party still able to leve a voice mail?

I have a land line and I do NOT understand why blocked numbers are both still able to call AND leave a voice mail. Can someone please explain to me how and why this is happening? 

Also, I get non-stop calls from "Unavailable 1" on my caller ID. How can I block this from calling me over and over and over again if there's no number to block? 

Can someone PLEASE explain to me WHY call blocking is not working and how to fix this annoying problem? I'd LOVE some peace and quiet again from the phone ringing non-stop. 

Thank you. 

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