UsingViber if you need to make a call to a friend who is not using viber app, then you need to have viber credits. To get the viber credit-
1. open the app and go to contacts. There will be an option to buy credits as it is marked in the screenshot. Touch the buy option.
2. Then you need to choose your pack. how much minutes you need to buy. It costs according to amount of minutes as you can see in the screenshot that 4.99$ will get you 216 min of calling.
3. After choosing your pack, the payment option comes. You need to fill up the forms and pay the amount to get viber credits.