I don't know about Viber but maybe what I found will apply for you. I was experimenting with a couple things, switched kernel and went back to stock, switched rom and didn't like so went back to energy and stock kernel. After this last change my wifi calling kept force closing within a minute of being on a call and I couldn't figure out why as I wiped and reset each time I went back to Energy.
Then I got to thinking, each time I re-flashed I also used Titanium to restore my apps.
So...I just went to manage applications, closed wifi calling app and erased data and cache for it. I've only used it a couple times since but both times it worked fine. Maybe if you restored using Titanium you could try this and see if that helps?