This has been happening to me too on my Galxy Note 2. When some says they hear a "beep" I look at the screen the number pad is showing that my cheek has been touching numbers. So my touch screen is not locking during th calls. I also don't hear the beeps but the person I talk to does. I will answer MARCTRONixx questions
what is "keyboard tone"? "BEEP" SOUND
do you mean the alphabet keyboard like swype or the "qwerty" keyboard or do you mean the phone dial pad? phone dial pad PHONE DIAL PAD
does this happen to other callers or just one caller? MORE THAN ONE CALLER
is your screen ON when this happens? does your screen turn on and off when you move away/to it? (the sensor makes this happen) IT MUST BE ON, I never paid attention before
do you use a screen protector? I RECENTLY TOOK THE SCREEN PROTECTOR OFF, HAS BEEN HAPPENING SINCE
is it in a case? YES
maybe I nee to put a screen protector back on? Will that make the sensor work better?