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Due to the way iOS works apps can not replace the main phone app so no. However in iOS 7 which should be coming out in the fall it includes call blocking so you have that to look forward to.
You can actually use the "do not disturb" feature built in to iOS to silence unwanted calls. All you have to do is enable "do not disturb" in settings, and then set it to only allow calls from your contacts.
Alternately, you can add each telephone number that you get unwanted calls from to your contacts, then set a ringtone that's inaudible so that you don't hear when they call. This is less elegant (and would require you to also get a ringtone that doesn't make any sound), but might be better since you'd still get calls from people who aren't in your contact list.