Viber
From NixOS Wiki
Installation
Install the viber package.
Wayland
Missing window
The app may not launch properly on Wayland - the tray icon will appear though there will be no way to show the window. To fix this set the QT_QPA_PLATFORM environment variable to xcb.
Scaling
If you have a HiDPI screen, you may want to scale the app. You can do this by setting the QT_SCALE_FACTOR environment variable to an appropriate number (e.g. 1.75).