This is the way it's implemented in Viber. Applications don't have to follow the standard calling procedure while you use a CDMA or GSM SIM card. It's totally fine even if it's says Ringing or Calling. I've just tested this on my Viber application and called someone and they said I was Calling, or at least that's how I appeared on their screen.
If you need additional help with the Viber application, I suggest consulting the online manual available for this purpose.