Just like on other VoIP concepts, the only thing you are paying is data bandwidth. If you are connected to Wi-Fi, it is free.
If you are using a 3G/4G connection, the final price depends on your data plan.
During the call, Viber spends approx 500 KB per minute (250 up and 250 down). So, if you have a 250 MB data plan (which is one of the smallest), you can gain yourself up to 500 minutes for calls.
Sending a message uses less than 1 KB. With the 250 MB plan mentioned above, you can say that text messaging is indeed totally free, even on 3G.
But with new feature called Viber out, released in November 2013 in Viber 4.0 version (for iOS and Android users), you can call anyone, anywhere, even if they don’t have Viber. In that case, you have to buy credits and we can not say that ALL calls are totally free but Viber to Viber calls are always free. Read more about Viber out call rates…
Anyway, due to the fact that Viber uses your data connection and not your GSM connection, your mobile service providerdoes not have access to your Viber usage. Any Viber and Viber Out calls that you place will not show up on your phone bill.