As far as I know, when someone blocks you on Viber, you lose the privilege to call that person, but he still can call you. Unfortunately, I believe this works only on Viber because I've used WhatsApp and it doesn't apply there as well. I recommend you place a regular call to the user to inform him about this issue.
You should know that when someone unblocks you, some time has to pass before you can actually search him or get in touch with that user.
