I need an app that unblocks blocked calls

No such thing.

Talk to your phone carrier. Blocking and unblocking is a carrier function.

You need a service like this:

http://www.trapcall.com/

Not cheap.

I think what you're describing is that you receive phone calls from a number that does not provide its identiy and you want to block the nuisance calls you're receiving from that number.

Blocking calls is a carrier function. Contact your cellular provider to find out what service or assistance they might be able to provide.

truecaller app free from app store worked for me