I have Nomorobo on my landline and it has worked very well up until recently. Now calls get past it several times a day. I simply answer and don't say anything, and then hang up. My theory is if they don't hear a voice (either real or from a voicemail greeting) they won't call the number again. Of course I have not idea if this actually works, but I figure it can't hurt and I don't have to listen to the phone ring a bunch of times before it goes to voicemail.