That is one of the things I like about viatalk. You can blacklist numbers. You can also have numbers that are allowed to ring. You can for example say that between certain hours all calls go to voice mail except a list. You can also pattern match on the name/number. So you could route all 800/888/866 and unknowns to voicemail.
You can also set numbers to get a recording to remove from list and you do not want their calls.
You can even have numbers forced to answer questions to prove they are not a computer. For example you can have all the people on your white list go through. Anyone else has to answer a question (they say punch in a code - if they do not punch in the code they are not allowed to ring through).