Better approach to handle 2 people calling simultaneously

Hello currently i’m facing a problem when someones tried to call another one and the other one make the call at the same time, the problem here is that when i check if there is already an incoming call before making a new one is not registered yet and the notification of the incoming call does not get instantanly neither.

Someone had experience a situation like this one or know a good approach to handle this problem?
Thanks!