OP, here's my suggestion (contrary to popular belief, I too was 15 once 😉):
Start doing odd-jobs around the house, other places, and see if you can't pick up a couple hours slinging burgers or whatnot. This does a few things -
1 - It shows you the value of money, makes you realize how many hours of work actually goes into one month of SMS and on-the-go web surfing via a phone.
2 - It will force you to save up for said phone, and who knows, once you have the money saved up you may find that you don't really want that phone at all, you'd rather put the money towards a car or something else.
3 - It shows your parents you have initiative and are serious. I cannot stress how important this point is. Us parents are weird people. We see our kids busting their posteriors for something they want, and I'll be darned if we don't see that and pitch in when you least expect it.
Good luck. Just try and keep it all in perspective. At the end of the day, it really is only a phone.