Need Some Apps

There are no blacklisting apps for stock iPhones. Your carrier may have blacklisting options for you though. Otherwise, your only alternative to block calls would be to jailbreak your phone and purchase either iBlacklist or mCleaner through Cydia. (The jailbreak would be a tethered one, since no untethered JB exists for the most current firmware.)

I personally use iBlacklist to block all calls from numbers that are not in my contacts; it sends them all to voicemail without ever ringing the phone. It hasn't missed a call yet; however, my phone is somewhat less stable since installing it, with more frequent Safe Mode crashes. It's still been well worth it.