Hi guys,
This is a member of the Viber R&D team.
I'm here to offer my assistance.
Viber is free between Viber users, doesn't matter where you live or what device you have.
Viber itself is totally free, meaning we (Viber) don't charge anything from you.
Since Viber uses the internet, both sides of the conversation must be connected to either WiFi or 3G. If a user is connected to WiFi - that user is not paying anything for sure. If the user is connected to 3G, the user *may* be paying his cell company for the data (about 0.3MB per minute of call), but that depends on the specific data plan that he has with his cellular company, and needs to be verified with them.
* Note that when a user is abroad using the cellular network (and not WiFi), there may be high Roaming charges by your cellular company for the data used.
If you have any additional questions, feel free to contact us.
Thanks,
Viber Team