I expect it is not possible to receive and recognize the incoming DTMF signal and display it on the screen with app inventor David. Do you have an example where someone has done this with a professional compiler?
Or an example where someone can record a phone conversation with App Inventor? See this discussion.Sound/Conversation Record During Phone Call (Both-side)
The above link and the discussion leads me to believe what you hope to do with App Inventor is currently impossible.