How to unblock the caller phone number? Or: how to stop forwarding calls from given phone number?

Current state

I have two phones, A and B. From A phone I can call whoever I want. From B I can call whoever I want except A -- when I call A, the call is forwarded automatically to voicemail.

It is not about network range, or something, or not enough money. It is most likely B phone number is blacklisted on phone A and such call is blocked (forwarded).

How to unblock it?

And what happened

I suspect that some call blocking app made that mess -- to test it, I actually block phone number A on phone B, and made a call. With the exactly same results as described above.

After that, I removed that phone number, than I uninstalled entire application, I rebooted phone, I checked the system settings (stock blocking: off, blacklist: empty). There is no other blocking app installed (just in case I removed related calling apps, like Skype).

Yet, once blocked, the phone number remains blocked.

How to undo this?

Phone A (the one with such permanent block) is Galaxy Ace 2, Android 2.3.

Updates

Update 1: On phone A in logs I can see all those "unreachable" calls. So they are received. Also there is small tooltip on phone A "forward call" (when call is received).
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).