How to block unwanted sales calls on iPhone 3GS with iOS 5?

Tried to find something in app store but no luck

Set up a do not answer list and add to it. Purchase a ring tone that says you don't want to talk to these people. I've tried and found no other way. I asked AT&T if I could set up a list of approved numbers or require a special code vpbefore cell will ring but it's only available on land lines.