In lieu of a proper tweak to accomplish this, I've found that if my screen is password locked, I can answer an incoming call and then press the sleep button to lock the phone while on a call. To make outgoing calls, I leave the phone locked and use Voice Control. Once the person answers I can then press the sleep button to lock the phone.
I realize it's a little complicated, but it beats face-dialing and face-muting while I'm on the phone.