It is the touch screen problem during the call.
When you use a bluetooth device or handfree device to answer a call or make a call, the screen will be on, then may touch anything, such as end the call, mute or hold.
Although there have a light sensor, but i dont know why keep touching anything in my pocket.
After i search information about this, i find "After Call Locker", but it only lock the screen after the call, not during the call, so i do it.
My app can screen off(lock the screen) when you make a outgoing call or answer the call.
AUTO CALL LOCK SCREEN(Free but advertisement, Displays only when you open the program to do a configuration)
AUTO CALL LOCK SCREEN PLUS(paid but no advertisement, because of the google service library, so this one file size are very less)
Sorry and hope you can understand my poor english.