iphone doesn't solve it... XMPP apps on iphone just use a server to be your agent and send push notifications instead.
- iPhone rings, Samsung nothing
- I wait few minutes just to be sure and pick up Samsung - immediately after picking it up the notification shows up, with the exact timestamp of the moment I picked it up
That's why I called it a tangent to XMPP
So the iphone version of any xmpp app has to use apples notification service, but the android version of the same app might try to use a direct connection (saving the app developer a lot of server costs), even though on many models of phone it only works when the device is charging for example.
I'll phrase my issue differently: Is there any way to have reliable notification delivery that are time critical? Is actually calling the only 100% reliable option, as the PagerDuty does?
Note that you can only have one push notification reliably per user interaction. So once you have sent a notification, further ones won't be reliably delivered until the user interacts with your app in some way.