The other part of me is worried that if I cut her off that she will go down the toliet, (she is finishing her college degree).
As Utopia said, you can't control her or cure her and you aren't causing her to behave this way. If her life goes down the toilet, it is because of choices she makes with no input from you whatsoever. You can't prevent her from behaving the way she behaves.
The best you can do is try to insulate yourself from any damage she does to you in the process of whatever she's doing with her own life. Usually, that means setting some significant boundaries, such as "if you leave a drunken voice mail, I will not call you back. If you call me and you're drunk, I will hang up on you."
After you set those boundaries, what *she* chooses to do with them is *her* issue. You are taking care of yourself. Walking on eggshells around her will not make her make better decisions about her life. Nothing you do or don't do will affect the decisions she's making.
I have a 'drunk dialer' in my family also. There have been a few times that this has nearly resulted in no contact because they were mad at me. But if they get mad at me because I won't answer the phone when they're drunk, that is not my problem.
I would rather be without family, than be with family who drive me insane. It sounds really harsh, but it is honest. I need to stay sane for myself and for those around me whom I love and who love me and who are not crazy-making.
You're worth being sane. You deserve to be sane. I suppose I finally realized that I couldn't really "lose" someone who was already lost to me due to alcohol. So in reality, I had nothing to lose, and my sanity to gain.
I can relate to your situation, and I certainly understand. It's a hard step to make even to accept that the person is already lost. Setting boundaries is even more difficult.