1. Allows an instant, full-screen foreground takeover over whatever else you are currently doing on the device
2. Rings and vibrates your device
3. Has a button that could allow an unknown person to send and receive audio to and from your device
4. All of this is triggered remotely, from anyone in the world, without any kind of user identification or authentication, besides a spoofable number
No app store's rules in either major ecosystem would allow such an abusive app. Yet, only because the legacy concept of a "phone call" exists, not only is the app allowed, but it's preloaded on every device out there!