Just like on other VoIP apps, the only thing you are paying when you make a Viber-to-Viber call is data bandwidth. If you are connected to Wi-Fi, the call is completely free.
If you are using a 3G/4G/LTE connection, the final price of the call depends on your data plan.
During a call, Viber spends approximately 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 will have 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 virtually free, even on 3G.
Viber-to-Viber calls are and will always be free. But now with the new feature called Viber Out released in November 2013 on Viber 4.0 for iOS and Android users, you can call anyone, anywhere, even if they don’t have Viber. In this case, the calls are not free; you have to buy credits to make these Viber-to-phone calls. Read more about Viber out call rates…
Because Viber uses your data connection and not your GSM connection, your mobile service provider does not have access to your Viber usage. Any Viber and Viber Out calls that you place will not show up on your phone bill.