Well, first of all the cell phone is the least of your worries. Almost every cellular phone company has pre-paid phones, get her one of those...then you're able to monitor her minutes. She'd get a new phone. Problem solved. I sense a tone through your text. you need to make sure that isn't coming through to your daughter...whether you like what dad is doing or not you need to bite your tongue on this one.
Take your daughter to a counselor. Get her someone to talk to who is neutral, so she can work through all her feelings without the worry of offending either parent. Children need someone to help learn how to deal with their feelings, and if a parent is not able to do this in an unbiased way, then it's that parents responsibility to find someone who can.
I am not in any way saying that what dad is doing is right!! IT'S NOT!!! Don't misunderstand me...but your responsibility is to your daughter to teach her how to deal with the situation the way it is. People are not always going to do what is fair, and the earlier a child learns to cope in a healthy way the fuller life the child will live.
And even tho you think you may be her best friend and you can talk about anything you'd be amazed how a neutral party can help...and if your daughter wishes, you can find a counselor that will allow you to set right beside her IF that is what your daughter wants, but I would definitely incourage her to do it alone expecially after she becomes comfortable with the counselor...so she can say whatever is on her mind without worrying about your feelings in the process.
The counselor will explain to her that her feelings are completely normal, she has every right to feel those feelings, and that this is in no way her fault.
Sometimes the hardest thing to do as a mother is give control to a professional expecially when it comes to emotional situations but it really is the best solution some times. This will not separate you and your daughter, you will still be as close as ever but it will show your daughter that you support her!!