Multiple pending APNS notifications delivered at once when the app enters foreground

I am working on a VoIP based PTT app. Uses 'voip' apns notification type to get to know about new incoming PTT call.  When the app is in foreground, the app gets to know about incoming VoIP PTT call through incoming push notification. After receiving notification, the app receives audio from the server. This works properly, as expected.

The app faces issues in receiving apns notifications only when the app is in background. Issue: My app is in background and iPhone locked, the server sends apns notification to the target iPhone about a new incoming VoIP call. Server received 200 success response code from APNS. But the app in background did not receive the notification. Second time, the server sends notification about second VoIP call. App did not receive notification. Third time, server sends notification about 3rd incoming call. This list goes on. Everytime server sends a notification to the target phone, success response is received from APNS. 


Now, bring the app to foreground, the app receives all the pending notifications at once. This scenario happens only in a few phones. Those customers are missing PTT calls.
Can you please provide us suggestions to solve this issue ?

My app uses unrestricted-voip entitlement

Multiple pending APNS notifications delivered at once when the app enters foreground
 
 
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