Issue with face dialing

[Vapeguy]

I keep having a problem of dialing new calls with my face while I'm on the phone. I'll be mid call and all of a sudden I'm dialing someone else's number and cutoff the person I was just talking to.

Is there someway to lock the screen while in a call so I have to use a slider before accessing any other call functions? I thought the prox sensor was supposed to turn the screen off when the phone is against your head. Maybe after all these years I just don't know ow to hold a phone to my head.