Which value 'apns-push-type' is appropriate for VoIP push?

I could see there is a new required header when sending push notification from iOS 13.


https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/sending_notification_requests_to_apns?language=objc


But I have no idea which value is appropriate on VoIP push between `alert` and `background`.