Hi,

This is a member of the Viber R&D Team!

Excuse me, My German is not so good so I will write in English.

Viber itself - the app and the service - is totally free, meaning we (Viber) don't charge anything from you.

Since Viber uses the internet to function, 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 cellular company for the data (Viber uses 0.25MB 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.

Please let us know if you have any further questions :)

Thanks in advance, Viber Team