The function of Viber allows you to use your real phone number to communicate with other Viber users. Viber scans your phone book and adds contacts that use the service to your lists allowing you to chat with them outside of your normal network.
While other versions of the app allow for phone calls, Viber on Nokia S40 only supports messaging and picture sharing. There's also a feature that allows you share your location through the app based on your GPS positioning.
Viber for S40 supports group chat, allowing you to exchange text messages with up to 15 people at the same time.
Although it lacks voice calling support, Viber for S40 makes a cheaper alternative to using SMS and MMS on your phone.