WhatsApp and Viber both allow audio and video calls. These two apps offer group calls and chat options. Both can be used for connecting through mobile phones with the cell number. WhatsApp can only be used when another person has WhatsApp too.
In contrast, you can make calls on a landline or another phone number not having a Viber account. This is the “Viber Out” feature by the app. Although there is a need to subscribe and you have to pay a particular amount but this feature makes it different from WhatsApp.
Another feature that makes viber different from WhatsApp is the game option. You can play games via Viber but WhatsApp lack this feature. Moreover, the Viber is known to be more secure as compared to WhatsApp.