Not receiving calls notifications

I am not able to receive notification of sending bird calls for the first time,

Here is the explanation
Each time when someone is calling to (let suppose) to me. I will not be able to receive calls notification if I have not started the call with someone yet. If from both sides of the call is started. Then notification is coming to receive call well and calling also working well.

I have checked for getting token saved, It was saving token very well. But not receiving notification. Please help me out, The app is to get live as soon as possible. everything is done, Only problem with Calling