FreedomFighter, the problem you are having here is only tangentially related to getting phone numbers. I'll try to help you see it another way. Forget for a second about whether you get her phone number. Think instead about the quality of the interaction. As DeepBlue would say, it's not getting her phone number but having her anxiously await your call that counts.
Many guys get caught in the trap of thinking that BECAUSE a woman gave them her phone number that she MUST be interested. As your own experience has demonstrated, nothing can be further from the truth. Often the woman is not particularly interested but when you ask for her phone number she finds it much easier to give you her number and flake later than to endure the discomfort of not appearing to be a nice person at that moment. Guys will often get mad at women because of this or view this behavior as deceptive but I see it as human nature to be prone to reacting this way in an uncomfortable situation. I think MOST people want to appear nice, friendly and agreeable even when another person clearly has something different in mind than them.
So with all this in mind, FreedomFighter, I would encourage you to take the phone number issue out of your evaluation for a moment and think about issues such as attraction, rapport, body language, frame setting, conversation and examine what is going on in THOSE areas.
There actually are some other logistical issues about phone numbers which are perfectly valid but my first take is to think that what I wrote above is closer to the problem you are having right now.
-PDX