[heyjudy]

NoMoRobo is free for landline VOIP in the US (maybe Canada too). Works like a charm in 98% of the time.

For cell phones, you'll usually have to pay for such an app that can intercept calls and check RBLs of naughty numbers.

[nikanj]

Checking for naughty numbers is completely useless, when the incoming caller ID is spoofed every time.