How do I get an iPhone 7?

For background, I m 17 years old, with a part time Job and on the T mobile family plan. As you know the IPhone 7 is around 32$ a month I believe. I make more than that in a week at my job, and I have enough money to pay the tax on the phone in store. But my parents won t let me get the phone because it ll add money to their bill. My dad believes it ll mess up his credit (which honestly is something I don t understand and would love to be explained) Getting my own plan is out of the question as It s not in my interest at all to do that or saving up to buy it out right. What are my options? I don t mean to sound like a brat but I think this is completely unfair.