Google Voice already puts calls like this into spam, and they never make it to your phone. Then you can go one step further, and BLOCK the call via Google Voice, then, when that number calls back, they get a pre-recorded tone/verbal "this numbers no longer in service" Loop recording played at them until they hangup. Either way, if the caller is going to your spam folder, or is full on blocked, they never ring your phone, and the only way you'll even see activity from them is if you go searching for them in your Spam/Blocked folder. Everyone who can should get Google Voice and use it, it's sooo nice, and it's great for more than just spam/blocked calls...you can always disable/uninstall it if you decide it's not for you, and it costs you nothing to check it out.