i have an iphone 12 pro max and the most recent ios update and my green phone icon just disappeared. help

Try a "forced" restart of your device using these instructions: Put it on charge for at least half an hour, then, keeping it plugged in, use these gestures:

Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the side (power) button. Only when the Apple logo appears (it might take 10-20 seconds), release that button (ignore all other screens that might appear, the one with the "slider" to turn the device off included).

If the icon doesn't appear, since it's not possible to delete that built-in app, check if you accidentally moved it to another screen page, or into a folder that you created.

Shouldn't be this case, open your App Library and find out the phone icon (it might be in "Social" category) then press and drag that icon to your preferred screen page (generally home page).

Regards

Giulio