I think it would be okay if it was a highlighted feature shown to the user during set up or a dismissible notification every month or so instead of a surprise when calling.
I hope you're joking, the last thing I want is pop-ups in my phone app. If the issue being solved is someone "forgetting" to press dial in a panic after entering 911 (which seems unlikely given that this is muscle memory ingrained into everyone), there's probably better ways to do the same thing. You could only auto-dial if someone holds up the phone to the ear. After a few seconds you could vibrate the phone and draw more attention to the dial button. Anything that doesn't break the usual muscle-memory flow.
They could add it to the WiFi calling warning notification that emergency calls are unavailable. It even has a do not show again button that fails to dismiss or prevent the notification.
