what is "keyboard tone"?
do you mean the alphabet keyboard like swype or the "qwerty" keyboard or do you mean the phone dial pad?
does this happen to other callers or just 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)
do you use a screen protector? is it in a case?
maybe some silly questions but need to get a good idea on your setup since we can seee it.