There is currently no way of doing this without concerning your carrier currently. But coming with iOS 7 (an update you can download this fall 2013), there will be a native way to block phone numbers.
Chat with our AI personalities
There is a Commcenter patch for iOS 5+ available for the iPhone 5, iPhone 4S and iPhone 4. This makes it possible for the user to edit the carrier.plist.
call the service provider and give them the number
Confidentiality and security are important when dealing with callers because you never know who is listening to your conversation. If you are not careful, someone may be able to use the information you say to steal the person's identity.
Jeff Daniels (2012 iPhone 5)
There is no possible way to call back a blocked number,unless you call your local police station a serious matter of course. But if it's not causing any physical harm just block blocked callers from calling you,by pressing *77 and follow further instructions.