Hm, I dunno. If it was me, I'd tell her totally seriously that it wasn't going to happen until some random time after she stopped bugging you (and maybe everyone in the family) for one and also have a serious talk about how the phone is the cheap part, paying for the calls is not, just so she's clear on it. Unlike you, I wouldn't make it a "back to school" thing because then you would have trained her that bugging people for 6+ months is the way to get things. But that's just me, YMMV.