I think that what you done was great. What i good starting point.
I know when i get phone calls, it always depend on the type of phone call. For example is she angry, sad etc.
When my mum is sad i tell her things like, "im sorry there is nothing i can do for you" or "look this is out of my control" etc.
If she is angry and abusive i say things like, "i dont have to listen to this, so im going" or " i havnt done anything wrong and dont deserve to be treated like this" etc.
What happens for me is i will feel guilty i am not there for her. I would like to help and do something for her but there is nothing i can do. She wont hang up the phone so i have to hang up.
It is really difficult to simply hang up when they go on and on, but it is for your sake you are doing this.
It will take her a long time to realise you are not going to take on her needs anymore, so repitition is key. After she kind of gets the point that you and not tolerating this anymore she will begin to lower the times she calls. Also she will know what mechanisms you put in place to protect yourself so she will expect how you handle the phone calls.
I also found with me that hanging up became easier to do after a while. When she is crying to me i put the phone down beside me and the odd time pick it up and say "yeah". I dont know how practical this will be for you but just telling you how i handle my mother.
You will begin to find your own ways of dealing with her once you learn the basic skills. So keep at it...
Stay strong and things will get better for you