It's not possible with an app, unless your jailbroken than it is possible.
If your not jailbroken, I advise you calling your cell phone company. Report the problem.
Overall there isn't much anyone can do because you can't really block a "Unknown Number" because it's unknown. I would jaibreak, I had an app when I was jailbroken that allowed me to block certain numbers from calling and texting me also blocking unknown callers from calling you.
Remember that jailbreaking is at your own risk, 98% of the time nothing bad happens, something might happen but you just over react and it turns out to not be that serious.
Goodluck.