This is an official representative from Viber
Viber to Viber calls or messages are absolutely free when selecting the Viber badge in your Viber contact list or the button which says 'Free Call' or 'Free Message' within the contact info of your Viber contact list.
All you need is an Internet connection: 3G or WiFi wherever available (3G may incur operator data charges or internet access fees. Check with your 3G provider to see your 3G plan). The bandwidth rate during a Viber call is approximately 240 KB per minute / 14 MB per hour.
If the party you are calling doesn't have an active internet connection or has their Viber application turned off when you call, the call will time out, and once the person reconnects to the internet, or once the person opens the Viber application, they will receive your missed call notification or the text message you've sent. Viber will not automatically divert to your regular GSM (cell phone provider) service.
If you have any question about Viber - please feel free to ask.