How to convince my parents to get me a cell phone.?

I'm 15 years old and I really want a cell phone. I'm in track and soccer at my school so I'm often at school after practices and sometimes we get out early. I also snowboard at my local ski resort a lot in the winter and sometimes my parents just drop me off and I would feel better knowing I had a cell phone to call my parents if I got hurt. I also would like to have it because it would just make me feel safer in general all the time incase i got hurt or something else bad happened to me. I even offered to pay for the phone and the bill too but my dad won't let me. Please tell me some other things I could say to try to convince him to let me get one. Thanks in advance.