I have an iPhone 5S and this is how I fixed the problem:
Click Settings
Click Notification Center
There are multiple sections which are separated by a grey tab, scroll down to see them.
The tabs are Access On Lock Screen, Today View, Notifications View, Include and Do not Include
If your viber app appears under "Do Not Include" grey tab, click it to open the settings screen, then follow the instructions below:
For Alert Style, there are three options: None, Banners, Alerts. Select Alerts to see the any alerts on the locked screen.
For Badge App Icon, slide the button to right to turn it on
For Sounds slide the button to right to turn it on
For Show in Notification Center slide the button to right to turn it on
For Show on Lock Screen slide the button to right to turn it on
Once done, click the Back button on top. Please note that there is no save button. Your settings will be saved when you click the back button.
Now your Viber app should appear under the Include Section and you should be able to see the notifications and hear the phone ring when your screen is locked.
Hope this helped. :smile:
Good Luck
Cheers