Just reinstalled Viber, but I cannot call my previous Viber contacts on free Viber, only to Viber out.
Viber uses the built-in contact list to detect the users who use the service as well. If you want to add a contact simply connect to the Internet then open Viber and click/tap the button with a figure and a plus. Enter its phone number and you should have the possibility to chat with the newly added contact. The Internet is required for this process since the application will connect to Viber servers to see if the user uses the service or not.