Hey man. You asked the question, so I'm expecting that means you want the REAL answer. So, do some yoga or whatever to get your breathing under control, and then open your mind as wide as you can manage, and consider the small possibility that you might, maybe, be a little bit wrong about some things, some times.
I'll give you a moment..
Ok. So, the title of your question was "My parents will NOT let me get a phone. How can I get one?" So that's what I'm going to focus on answering. The description was mostly just you venting to the internet, which you're allowed to do, although I wouldn't make it a habit--stuff like that can come back to haunt you.
1. Most importantly, respect your parents and be kind to them. You won't be able to comprehend this right now, but your parents are the most genuine supporters you will ever have. There is almost nothing substantial that you can do for them, but they don't care; they just love you, and absolutely anything that could make your life better long-term, they would do.
If they are not willing to do something, it's because they are not convinced that it's the best option for the you that exists right now. Also, if you act like a brat every time the topic of XYZ comes up, then they'll actually be obligated to say no, even if they want to say yes, because teaching you that 'pestering and making weak arguments and empty promises' is a good way to get what you think you want would be irresponsible of them.
So, man up and do whatever you have to do to keep your attitude out of conversations with your parents. The saying "you catch more flies with honey than with vinegar" simply means that being sweet gets the job done way better than being nasty. This applies to nearly any situation. No person has ever heard a mean or sarcastic comment and then replied, "you know what? Your bad attitude and rude remarks really changed my mind. Here's what you were asking for." If anything, a good argument is crippled by the tone, manner, and timing in which it's delivered.
You're creating a person to be every day, and then you're practicing playing that character over and over. You want that person to be someone that other people want to be around, not just because it's right, but because it's actually best for you and puts you in a better position to reap benefits, like approved requests.
2. You've obviously considered the initial cost of the phone. That's good, but have you considered the monthly cost of cellular connectivity? When you offered to pay 100% for the phone were you also implying that you would cover the monthly cost? What recurring income do you have that's sufficient to pay for cell service? If you have a recurring income at 9 years old that's not just a handout from your parents, then that would be very impressive indeed. Anyway, always remember recurring costs, taxes, and fees associated with things in general.
3. What to do:
So thing 1 is: you need to make your main focus being the best person you can be, being content with what you have and where you are. Don't think, "what can I acquire for myself that will make me happy?" [spoiler: it won't]. Instead think, "what is it that I can do right here, right now, that will make the current situation better for the people around me?" [you'll be surprised at how happy this actually makes you yourself].
Thing 2 is: your parents have to want you to have a phone. It doesn't matter how much you artificially feel the "need" for a phone and put yourself through emotional turmoil over it. Your parents have to be having experiences that make THEM think, "hmm...if our kid had a phone then it would have been helpful..." and they have to have those moments more than once per month. You will not be able to just describe the possibility of these moments to them or desperately argue them into existence; they actually have to happen. Most of these moments will happen naturally over time. You don't have to try to generate them.
So the less you are stressing about getting a phone, the faster it will come about. The more you're fixating on it, the more likely you are to actually be standing in your own way. Also, phones are a major factor for many people contributing to low productivity, low happiness, trouble sleeping, excessively comparing themselves to other people, getting their personal data stolen/sold, fraud, cyber-bullying, and more. For every benefit to having a phone, there is at least one negative side-effect. My favorite times are always when I'm away from my phone and I'm enjoying a really cool project.
Finally, something for your parents to consider when they are ready is Gabb Wireless. They're supposed to address many of the valid concerns that parents may have with phone usage.
You'll be okay, and in a few years, you'll be looking forward to times that are free of electronics. People used to visit the internet to escape reality for a bit. Now people visit reality to escape the internet for a bit.
"Actions speak louder than words" is another cliche you can remember. Don't try to nag your way into more privileges. Work and act your way into whatever responsibility you can handle and privileges will show up automatically. Your parents don't owe you anything. The world doesn't owe you anything. But, become a great person and your parents and the world will be eager to reward that kind of noble effort.
You got this, man. Get after it.
