Assuming you were jailbroken, and using a Cydia app. If that's the case, unfortunately, there's no way back. Once you have updated your iOS you are stuck with it. You now have to play the waiting game and hope that the jailbreak for iOS 6 arrives soon.
I'm not jailbroken, and never have been, so I'm not familiar with this Viber thing that you mention. Was that your app for making free calls?
If you are in the US, Google Voice offers this functionality. It doesn't work elsewhere, though.