That's one of the reasons I'm not using iPhone 4 anymore, sad to say but true. Blocking calls doesn't need to be done on the carrier's side. Every other phone can block calls or buying an app just for that. On my phone right now (no need to mention it by name), there's an app that can block calls, by numbers, unknown, private and by manual input, blacklist and/or whitelist, from contacts, from call history and more. The calls comes through and you tell the phone what to do with it seeting it up for the first time, reject, ignore, go to voicemail and pickup and hang up, all this by location and times specified on the profile you create with the app, you'll never hear a ring but a notification that a call has been blocked if you want. Also will keep a log of blocked calls.
The ones that say that it needs to be done at the carrier level, IS NOT TRUE, well, let me specify, for the iPhone that's the case, not the rest of mobile phones on the market, shame.
Developers can't create an app for this because Apple will not release the API for their phone app, whihc is understable, again, let me specify, they have done it, but not allow on the App Store, shame.
There are many options out there other than letting AT&T just block 5 calls without any type of control, but that is not to be discuss here.
I still have my Iphone 4 as a second line but using a more controllable device, wish Apple will allow something like this on the App Store, will be my perfect mobile phone, cheers!