how to keep screen locked when initiating a BlueTooth call?

[mlevin77]

When my phone is locked (and paired with the car's bluetooth stereo), and a call comes in, the screen stays locked. This is great because I can answer it with my car's BT unit and never take the phone out of my pocket and no buttons get accidentally pressed. But, if I *initiate* a call from the car's BT unit, the phone app comes up and during the call the screen is responsive to presses even though it was locked (although it does go back to the locked state after the call is over). This is terrible because I've had stuff changed accidentally by my jacket's contacting the screen while it's in a pocket during a BT call.

Does anyone have any idea how to keep it in the locked (screen unresponsive) state when initiating calls by BT?