I think you did a smart thing to buy a prepaid phone and see how it went from there . . .
Do 12 year old girls need a cell phone? No. Do they use cell phones to "fit in with the crowd?" Yes. Only you as a parent can make the decision as to whether or not your daughter actually needs a cell phone. Sometimes fitting in with the crowd is important -- and if it is, and the cell phone usage is expensive, then having your daughter pay for it in one way or another is perfectly appropriate.
My 10 year old SON was asking for a cell phone last year -- and I said "Who in the world would you call?" and he said "YOU!"
Needless to say, as sweet as this was, I am not getting him a cell phone.
We lived in California in 2001, and EVERYBODY had a cell phone except my kids. I had my 15 year old son BORROW mine whenever he went somewhere, just for our convenience. He did not get a cell phone until college. Last year, we gave our 15 year old daughter a cell phone, again, for our convenience. Yes, EVERYBODY but her had one by then. And then, everybody BUT her had a cell phone with a camera in it. Now that our 11 year old son sees that his siblings have one, and some friends have them, he's interested in getting one as well. BUT, until he has activities that keep him away from home, I'm not planning on getting him one for a long time.
Kris