As mentioned by the first response..... iBlacklist
Device must be JB and it's $12 I think but TOTALLY worth every penny!! You can set up various blacklists as well as whitelists, and customize each contact in them.
For instance, I have mine set to intercept all "unknown" and "blocked" calls and instead of voicemail I have it give a busy signal. And since those horrid telemarketers now get cell phone numbers (even those on the DNC list) if I get a call from a number I don't know I ignore it and then typically google it and if it turns out to be a telemarketer I go in blacklist and add that number from recent calls and set it up for a busy signal.
Then there are those callers that you may want to leave a message, so you add them in there and set it to automatically go to vmail without ringing. It has a feature to automatically reply to a caller with a text message that you customize specifically for that number.
You can also block text messages from numbers in the blacklist. You set it up to be active during certain timeframes only. Or you can activate it to only accept calls from those listed on your whitelist!!!! Numerous different ways to set it up. It is one of my favorite apps!
I've had it for several years now, so I don't know if it's still this way, but when I first found it you could download it for free and try it out before purchasing it.