So this has happened to me twice.
Both time occurred before upgrading to 12.0.1
I have noticed in both occasion I was answering the call via BT. First time the phone was in black screen and I hanged up via the car hang up button. And the phone will not wake up from sleep. It took over 6 hours and trying different power source and cable- it finally rebooted using iPad Pro power brick and original Apple cable. I was told to enable use of USB for recovery.
Second time it happened- same thing in BT. The phone app also hanged. I can’t disconnect the phone call (ended up having my wife hang up). The phone app can’t be exited. Swipe up won’t work. Can’t turn phone off. Can’t reboot. Took over 2 hours to finally reboot. It just worked after couple hours. I was told to upgrade to 12.0.1. The problem hasn’t returned.