per Table 1 in https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/sending_notification_requests_to_apns.
The important part is that the apns-push-type header is: "Required when delivering notifications to devices running iOS 13 and later, or watchOS 6 and later. Ignored on earlier system versions."
Should it be background or alert if it is required for VoIP on ios13?