Cannot answer incoming Viber calls

I have iPhone 5S with iOS 7.0.6 and Viber version 4.1.1

Viber is running in the background and the iPhone locked.

Any incoming Viber call will sound the audible alert and there is a notice sayng there is an incoming call, but it oes not present the "Accept" or "Decline" buttons on the screen. When I enter the password to unlock the iPhone the incoming Viber call is cancelled.

When there is any standard incoming call the "Accept" button is shown even when the iPhone is locked.

How do I set iPhone or Viber so that I can accept incoming Viber calls.

Any help much appreciated.

Thank you. Briabn1939