Some mobile operators across the world block Viber on their 3G network. Usually this happens when users have a "simple" data plan (or a prepaid contract), meaning they didn't pay extra for using VoIP on their 3G network.
In order to verify this, we recommend to:
1. Test Viber on a few different WiFi networks, and see that it works fine.
2. Contact your cell company and confirm this with them.