If the other contact with Viber account is offline, it asks to dial directly to mobile number using your Viber credit.
Besides, if you have enabled two-factor authentication on iPhone, you will need to extra type in the random code.
This will recover all the selected deleted Viber messages to your PC or Mac.
Although the app is pretty popular and secure, users often complain about getting their messages deleted.
It just takes two steps to automatically back up your Viber messages and you don't have to worry about the case of forgetting to back up messages.
After the recovery process finishes, all selected Viber messages will be restored on your iPhone.