I have a Samsung Galaxy S6, and my dial pad doesn't work while I'm on a call. For example, I call in to a conf call, it asks me to enter my access code, and the phone doesn't recognize any of the numbers or keys I press on the dial pad.
I googled the problem, and the only fix that worked for some people isn't working for me (going into Settings>phone>DTMF Tones>make it "Long" setting).