Alphas don't act uninterested. Alphas act human. They act careless in other areas but not in your situation. In fact, an alpha in your situation would have already gotten that girl's number.

Look, you fucked up by not getting her phone number. That's history, you learned from your mistake. Now I want YOU to stop giving a shit about her and her IOI as well. Just when you see her next time...be like, "Hey you're that one girl..we met before." She'll say a few things back and then just take out your phone and point it at her heart so that she can grab it and say, "Put your number in." If you don't have a phone, get out a pen and tell her to write her phone number on your hand.

If she asks, "Why do you need my number?" Just say, "You seem like a cool girl" If then says, "Yeah and?" Say, "We rarely see each other. Look, if it bothers you then no biggie. Take care." Or if she comes up with some excuse just say, "Trust me, it's ok." Then if she gives you another excuse and you SENSE that it's fake for real, say, "You don't have to lie you know." It's pretty easy to tell when people are lying.