I don't know if a phone message "ultimatum" is a good thing here. This isn't some chick you just picked up, this is a girl who has been a friend of yours for a long time, and now things are moving to the next level. Your previous relationship with her makes a lot of the DJ tricks go out the window, mainly because you have already established trust with her, she probably knows a lot about you already, and vice versa.
You could LJBF her, but you already are friends, and how much of an ass would you seem to this girl if you made out with her, and then told her you weren't interested in her? You don't play mind games like that with girls you've known so long as friends. That will only serve to hurt and confuse her.
You called her twice in 9 days and she didn't return your call once. Chances are she's probably thinking about what is going on. She may be a bit scared, wondering if in the case that things don't work out between you if you two will remain friends. She may be wondering if you are actually as interested in her as you seemed when you were making out with her. She is avoiding you for some reason, and it's probably because she's scared of things.
Because you two are already friends the best thing to do in this case is to talk with her face to face about what you two are feeling and what you both want. Be open and honest about it. Although I would certainly not advise doing this with some chick you just picked up or are trying to set up to go out with, you may actually have to drop by her house or place of work unannounced and force her to talk about it; if you are close friends with her already this won't be "weird".
By the way, how old are you two?