Sorry, I'm going to be blunt here...your dh does not sound totally innocent in all this. Maybe he is just flattered, but saving her number under another name is not to save your feelings, more like to hide the calls.
It is unlikely that a man cornered is going to admit his guilt straight off, when he can say it is all the other woman's fault. It sounds as if she is really sure of herself in all this, so where is she getting that feeling from if your dh is not encouraging her?
Your dh needs to cut off ALL contact with her, because it is obvious that she can never be just friends with him, and it is not likely that you feel many warm feelings towards her!
I hope that I am wrong. Sorry if what I have said is too upsetting. I just don't think it is worth pussyfooting around