If your device is locked it should be impossible to butt dial as far as I know.
The only thing I could think is that maybe you are triggering active edge (the pressure sensitive sides).
If you look in settings > system > gestures > active edge you should be able to adjust the sensitivity, and other options like disable assistant or disable when device is locked.
I did notice that if my device is locked and I trigger active edge, it will bring up my assistant and I tapped make a phone call just to test but it requires me to unlock the device before I can make a call.