Why do some of my inbound VoIP calls end up on Viber?

You may receive inbound calls via Viber if the Viber-In Calls feature is enabled in your application. This feature lets users have incoming calls with Viber when possible. 

If for some reasons you don’t want to receive calls on Viber, it is easy to disable Viber-In Calls:

1. Go to Settings —> Calls and Messages in your application.
2. Click the Viber-In Calls toggle to disable it.