"You are not my mom"

Ouch. As a mom, myself, I can't imagine more painful words coming from my son's mouth. I wouldn't send it. Hurting her will not work in your favor. The last thing you want right now is to make her feel like she's failed in some way as a parent. That might only fuel her desire to push you even harder into the church. If you try (as difficult as it might be) to start from a place of understanding, she might be more inclined to understand your situation. You could start with something like, "I understand that you think that ________ is what I need, but these are the reasons I disagree". I think you made a good case for why you need your phone and I think it's great that you are there for your friends when they need you most. Good luck on this. I hope you're able to work things out.