@kuro0314 -
You need cellular reception only when registering, so we can send you the confirmation SMS with your code. After that, basically you can use your Viber on WiFi only. As HeartRoxas mentioned, you can also use it on 3G, but it uses MB from your data plan (0.5MB per minute of call). The amount you pay for your data is decided between you and your cell company, and has nothing to do with us. If you have a monthly pre-paid data plan, then you can use Viber freely without any concern of extra charges. WiFi,  of course, is always free (and usually better for Viber calls).