There is no way that Teresa could have heard Ellen dissing her through the door, because Ellen wasn't even near the door when she spoke the hurtful words, she was walking towards the door. Furthermore, she wasn't yelling, so even if she would have been near that thick wooden entry door, there's still no way that Teresa could have heard her from outside. At most, she would have heard vague noise, but couldn't possibly have made out what was being said. Yet, Teresa is evidently upset at Ellen, when the latter opens her thick front door.