Cross platform VoIP instant messaging software application
Rakuten Viber, commonly known as Viber, is a cross-platform voice over IP (VoIP) and instant messaging (IM) software application owned by Japanese multinational company Rakuten, provided as freeware for the Android, iOS, Microsoft Windows, macOS and Linux platforms. Users are registered and identified through a cellular telephone number, although the service is accessible on desktop platforms without needing mobile connectivity.
https://en.wikipedia.org/wiki/Viber