There is one called Call Blocker that is free and says is iOS 6 compatible.
Call Blocker is $0.99...not free!
Can you test to see if that works on iOS 6While researching, I found that there is an app called SMSNinja which is free. It supposedly works with MMS/SMS/Calls. It only lists iOS 4.0 to 5.1 though.
Can you test to see if that works on iOS 6