The phone icon on my 15pro Max is missing. Why did this happen and How do we fix it?

1. Can you find the App Library? Swipe you Home Screen from right to left. Keep doing that until you get past the last Home Screen and the words "App Library" will appear in the search box at the top of the screen.
2. Look for the Phone app there.

If you don't see it, can you find it by using Search?

The Phone app is not one of the apps that can be deleted but it can be removed from the Home Screen. Go to the App Library (swipe left past your last Home Screen). From there you can touch and hold an app and slide it to the Home Screen.

Also note the Search window at the top of the App Library.