Ending phone calls

It seems the bug occurs when you exit the phone app to use for the phone for other purposes while still in a phone call, and then try to return to the call via the standard built in green Apple IOS Phone app. When you return via that route, the phone doesn't seem to even know you are on a call and you can't hang up, and it is NOT obvious how to hang up at that point. (Aka, you get the 'green button' instead of the 'red button'.)

Don’t tell me to scroll up or reboot or go back to home page as nothing works! My partners phone has same issue….this is a security issue, Apple advise how this gets sorted ASAP. Been like this for months