Hell no and by saying that, you should realize she is just trying to manipulate you and think you'll be gullible enough to believe her and keep paying for it. I got a job while in high school and my car, cellphone, gas, car insurance...anything I wanted was something my parents said I could work, save up, and buy on my own so I had none of that stuff. I never made them pay for my phone nor asked for it because I knew the answer would be no. They wanted me to learn the value of hard work and although at the time I regarded it as mean, I ended up appreciating it.
As I worked and went to community college, I would be dropped off at school or my job and given a bag full of quarters to use a payphone if I could not find a landline, though my college class schedule was pretty much set in stone so my mother knew what time she had to pick me up, so really, no need for a cellphone. The quarters were in case the class ended earlier, or if I needed to stay later to work on group projects or attend tutoring. I got my first car at 21, made all the monthly payments, the down payment, car insurance, gas, etc., which is why it took a while for me to save up and buy, but I was very proud of myself for having worked my butt off...this was MY brand new car!
I got my cellphone a little earlier than that, paid for it myself, and paid my way through college, then moved out. I work with a couple of people still in their 20s and none of them have their parents paying their phone. They may still live with their parents because they are single and childless, but they buy their own clothes, food, pay their own car, and do household chores so 30 is beyond that point. Many people have families of their own by then and running a household. As to the concern of keeping her number, I have changed carriers about 4-5 times in my lifetime, starting over a decade ago, and during all that time, was able to keep my phone number. Even landline phone numbers these days can be ported, granted they haven't been inactive for years and re-assigned to someone else.