Advice?

Okay, So I Had This friend.
She recently started giving me prank phone calls, calling me a daft b.itch, and threatening me, screaming and swearing knowing my three year old sister could hear.
I don't know why she did this, we never argued or anything, when she moved into her new flat we were on good terms and agreed to stay in touch.
But then she did that and then she did it again a few nights back.
I told her I knew it was her, and she denied it, but I KNOW that it IS her, because of some of the things she said on the phone, there is no possible way it is someone else.
I'm not too bothered about the threats-I can take care of myself and she is all mouth anyway-I am bothered about the fact my sister is hearing the things she says.
I refuse to get a new number because I shouldn't have to do so, and she phones off a with held number, but I have to answer it because some of my other friends and family phone me from with held so it could be them.
I also refuse to hang up because then she will think she is getting to me, and I have tried taking the calls privately, but as my sister is only three, she doesn't know anything is wrong and follows me.
She is nearly 18, and still doing prank calls.
How can I get her to realize how pathetic she is, and stop?

Some friend [loads of exclaimation marks but mine don't work] I know You shouldn't have to change Your number but if I was You that's what I would do, she obviously doesn't care about You or how this will effect your sister.. I say change ya number and cut her off completely.

that is out of order the way she is acting, it is very childish if she had a problem she should address you in a much more maturer way thats what i think especially with her being nearly 18. she should grow up, but on the other hand have you done anything to her? said anything behind her back that sort of thing? im not accusing you of anything just wondering why she is like she is there has to be a reason right? hope havnt made you mad if so i'm truelly sorry im only being honest takecare mel xxx

Lol, I'm not offended, but no I haven't done anything to her, I just told her she was pathetic and needed to get a life when I realized it was her doing it.
Although since she found out I knew it was her, I haven't had any more calls, so hopefully she has decided to use her downtime to reflect and grow.
xx