It is possible that you encounter this behavior due to a network error. It's advised to always use a stable connection while trying to call using Viber. Also, it's possible that the person who you want to contact is encountering network problems. Also, this might be caused by an internal error of the application. To fix this, you should go to the menu of your phone, under Settings > Applications > Manage Applications > All tab, tap on Viber and Clear Data and Clear Cache. After you did this, try calling again your contact. If you still encounter the same error, advise him to do the same as you did, and if this doesn't help, contact the developer on the official website for additional support.