My buddy mct just created an awesome little piece of software called sipscreen which allows you to filter out VoIP calls based on rules of your own choosing on your Linux gateway.

Here are some example call policy rules:

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exit 1 if $name =~ /TOLL FREE CALL/i or $number =~ /^(800|877)-/;
exit 1 if $number eq "000-000-0000" and $hour >= 3 and $hour <= 9;
exit 0;

If you’re using VoIP and have it connected to a Linux gateway, you should definitely check this out. Its way easier to accept/reject calls than having to fire up a full Asterisk install.