How to unblock a blocked caller?

Does anyone know how to unblock a BLOCKED caller?

no, you cant! that's the whole idea.

If you want to do this because you're getting abusive phone calls and want to find out who it is, you need to call you're phone provider (in Australia, its telstra) and talk to them about it. there are procedures for prank/harassing calls, and they'll be able to ether tell you who it is or take you're details, so that if it happens again, they'll do something about it next time.