VoIP Notification: HTTP/2 Notification Provider API is giving timeout exception

Brief background about our application:

We are developing an iOS application where we are utilizing the PushKit and VoIP notification for showing the native Calling UI using CallKit to the end user for the video calling.

We have generated the VoIP certificate from Apple developer portal and we are using the same to send VoIP notifications from backend APIs.

Just for information, our application's backend is in .Net MVC.

Details about the issue we are facing:

The problem we are facing here is with the HTTP/2 notification provider API, which is giving Timeout issue (please refer the error log mentioned below).

This timeout error occurs intermittently. Timeout error is not encountered every time. Once or twice in 5 calls, we get timeout error. If we try again after sometime, it again starts working well, but same intermittent issue is encountered frequently.

Here is the exception details of what we are getting:

System.TimeoutException: The operation has timed out.
   at HttpTwo.Http2Client.<Send>d__19.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at HttpTwo.Http2Client.<Send>d__17.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at HttpTwo.Http2Client.<Post>d__15.MoveNext()

Things we have tried:

1. Keeping the connection open. (i.e. not closing the connection after every request)
2. We also tried using third party plugin (PushSharp with HTTP/2)

We would appreciate any help/support on this issue.