My iPhone screen stays on during calls. How do you get it off?

Every time in on the phone I mite the call or put it on speaker, it's really annoying.

If you don't want to see the phone button list while on a call, then press the Home button at the bottom of your iPhone.
That will take you back to your Home screen with all your apps while keeping the call alive.

If you want to get back to your phone button list, then there should be a small green bar at the very top of your phone screen that you can tap.