You can identify unwanted calls. I still have a land line, but I am forwarding all calls to my iPhone. To identify unwanted calls (new windows, green energy, political adds...), I created a Contact named Don't Answer.
When I answer the phone and it is one of those annoying calls, if the number shows up, I can just use the 'add to existing contact' feature. Of course, this doesn't work if the number is blocked. But, in two weeks, I have a long list of calls that show up as Don't Answer and I can just not answer or choose Decline—there's no guesswork involved.
Lex adds: I do something similar for another reason. I use Google's two-factor authentication, and Google uses a variety of phone numbers to text me my passcode when I log in. So I created a Google entry in my contacts, and add each different number the company texts me from to that contact. That way, instead of ending up with lots of old entries from Google in Messages, there's just one that all their texts get grouped in.
iOS 7, of course, will provide a way to block specific unwanted callers, too. But this hint makes a nice stopgap!
Mac OS X Hints