It sounds like she has very low self esteem, and certainly the phone issue could be her way of feeling worthy. The more social contact she has with the phone the more popular and wanted she feels. Then it becomes the be all and end all, and everything else suffers.
I do wonder if her treatment of you is some kind of reflection of how she feels about herself. I think it sounds like she has welcomed the new boundaries you have set in place, and perhaps when you were giving in to her demands it was counter productive. You thought you were giving her what would make her happy, when what she really needed was you to say no and put the boundaries in place.
I also think you are putting too much emphasis on the fact you are a single parent. I am not sure that is necessarily a reason for her behaviour. I imagine my behaviour at her age looked similar to hers, and I was brought up by two parents in a stable marriage. Yes, you had her young, and yes her dad left, and yes you are doing it alone, and you recognise that those aren't excuses for poor behaviour, but you also suggest that you have, in the past, parented her through guilt because of those things. I hope you don't feel guilty about all that, because you have no reason to. You sound like a lovely mum.
I think you need to hold on to the fact that we often hurt the ones we love the most, and children certainly rail against the people they trust to love them unconditionally. Try not to take it all personally (easier said than done I know).
She is going through a lot, she is struggling at school, and although it looks like it is at her own hand, she is possibly feeling like it is all completely out of her control and is insurmountable. It can't be easy to get up every day and go face a school full of teachers who have you labelled as difficult, peers who see you as a trouble maker etc. The boyfriend could be a welcome refuge from all of that.