How to unblock the caller phone number?
Found it!

The culprit is this app: https://play.google.com/store/apps/details?id=com.vladlee.easyblacklist Calls Blacklist

Listen to this: if you add any number to the blacklist of this app, and UNINSTALL this app after that, the number will be kept hidden and block. 

I.e. there is no way you can unblock the number, you can only do this by installing this app again, and removing all phone numbers, and then uninstalling the app.



To be of more general use -- if something similar will happen to you, go to Google Play website, check the history of installed apps (which blocks calls). Install all of them back, and then check their blacklists. Clear the blacklists, and then, with clear list, uninstall them. It should do the trick (hopefully).