When you receive a call through Viber, the incoming call screen will appear with three options:
1. Answer - To answer the call
2. Decline - To decline the call
3. Message - To decline the call with a text message
If your screen was locked before the call came in, you will see the incoming call as a banner on the locked call screen. To answer the call, swipe the banner to the right. If you would like to unlock your phone, but not answer the call, swipe the "slide to unlock" at the bottom of the screen. Opening the Viber application after doing this will display the incoming call.