I am unable to get Viber ringing on incoming call even when the phone is active

[AC Question]

I am unable to get Viber ringing on incoming call even when the phone is NOT on sleep.
Not only it is not ringing but also it is not giving any kind of sign, until the ringing person hang up )and then I can see that I have got an unanswered call)
I have checked all sounds and wi-fi settings, but everything looks fine.
Any hints? Thanks in advance!