The app would read your phone's address book (with permission) and automatically flag which contacts already had Viber.
The difficulty in porting Viber to J2ME stemmed from the fundamental architecture of the platform: Limited Hardware Resources Viber For Java J2me
: Officially, Rakuten Viber does not support Java J2ME platforms. Current supported environments are limited to Android 6+, iOS 15.2+, and major desktop operating systems like Windows 10/11, macOS, and Linux. Third-Party Efforts The app would read your phone's address book
On 2G networks, maintaining a constant socket connection for instant notifications was a challenge that required clever coding. How Users Installed Viber on Java Phones Viber For Java J2me