This is an official representative of Viber Media.
The source of this issue could greatly vary. Please read this short page in our HelpDesk, quoted below outlining some possible workarounds that will help reboot synchronization of the contact list:
Android: Contact list is not showing your friends as Viber users?
Make sure you are using the latest version of Viber. You can download the latest version from http://www.viber.com/
Make sure that you are looking at the Viber contact list, not your device's contact list.
In order for your contacts to see you as a Viber user and be able to call you through Viber, your phone number, which is saved on their address book, must be the exact same number that you used to register to Viber and vice- versa.
Try tapping on the 'More' tab at the bottom of the Viber app, then select "Settings" and the add a check mark [√] to "Show all Contacts"
Once you installed Viber, if you do not see someone in your contact list whom you know to have Viber, it might be that the list is not updated (this may be due to an unstable connection during setup, or just a slight delay). To update your contact list, re-sync it by re-initiating Viber or by manually updating your contacts list.
You don't need to uninstall and reinstall Viber. Shut Viber down (so that it no longer runs in the background) or turn the phone off, and then re-open it.
If this does not work, try the following steps:
Send the user an invite and if the number is a registered Viber user, it will automatically update the Viber contact list.
Enter into your Viber contact list, then select the device's menu button, and tap on "update contacts". Then just click your device 'home' button. Viber will run in the background. In about 30 min. to an hour, check again by clicking on the Viber icon.
Confirm that you have an active internet connection.
Dial a number manually, using the Viber keypad
Delete the contact with the missing Viber badge from your contact list. Add it again and make sure to include the country code.
The phone number format should include the plus sign, country code, cell code and phone number. The standard format is: +(country code)-(cell code)-(phone number)
For example: +1-555-555-5555
For more questions or technical support, please visit our official HelpDesk.
From the FAQ:
Try tapping on the "More" tab at the bottom of the Viber app, then select "Settings" and the add a check mark [√] to "Show all Contacts".