I have a bit of advice because I had a friend like this too and didn't know how to cope with her, until one day it all finally came to an end!
She actually cheated on her boyfriend and was seeing another bloke behind his back, and she would constantly droning on about how she didn't know who she liked the most, whether to break up with one or the other, etc. She would ring me to tell me how the one she was cheating with had whisked her off to a hotel for the wkend & how she came up with a cover story so her actual boyfriend wouldn't know where she was, how exciting it all was, blah-de-blah. Anyway her boyfriend found out in the end and it all came crashing down & then she was constantly sobbing down the phone about how she had made a big mistake, ruined her life, etc.
Anyway the day I finally lost it was when she rung me one day just after I found out I was pregnant. I listened patiently to her going on about her usual problems, until she finally paused for a minute and said 'Oh how are you anyway?' I told her that I was really happy because I'd had some brilliant news & found out me & OH were expecting a baby, and she said 'Ohh that's nice, how cute!'... before asking me to wait for two secs coz she had just got a text on her mobile from her ex & got all excited (at this point she was trying to get back with him, her 'original' boyfriend) and had to read what it said!!!! I told her to go and read it and not to bother ringing me back coz I didn't ever want to talk to her again, she acted all suprised and innocent but I told her I was sick to death of her going on and on about herself all the time and to find someone who actually cared! She never even asked how my daughter was, or what she'd been up to, etc., and I realised she wasn't a friend at all because she just wasn't interested in us! So basically (sorry for the very long reply
) I thnk you should just be straight with her and tell her that you feel she's not really a friend and you are unhappy with the way she acts and ask her if she can please stop ringing you. If it makes it easier, send a text to her saying that! Bit of a cop out but what can she really say to it? Then maybe change your number!