phone doesn't lock screen while on a call
After upgrading to ios 11.2.6, my sleep/wake button won't lock the screen if I'm on a phone call. After pressing the sleep/wake button, I see the lock screen, but any movement of the phone then takes me back to the Phone screen -- that is, doesn't seem to be a way to lock my screen when I'm on a phone call. Any ideas?
Phone: 6s, ios 11.2.6
iphone, Windows XP Pro