Are you just going to stop paying the phone bill or other things as well?
I think you are well within your rights to not pay her phone bill personally, or you could just pay the line rental and she pays for all the calls if you want to compromise.
I think it is a fine line between being her friend and also allowing her to miss you and feel some of the impact of what she is doing. She can't really have her cake and eat it.
I suppose what it will really be about is the way you broach it with her. If you want to support her at school then you could pay for essentials only and she has to pay for her 'fun'.
