Why is the phone number on my sim card different than the one on my account?

I recently changed tmobile plans. I was on the old prepaid and switch to the new prepaid. I was told I could keep my old number. After changing, my online account displays my old number as active. But the phone number showing on my device and when making calls is this new number. I have repeatedly called customer service (4 times so far): they keep telling me it is taken care of and to restart my phone in 2 hours. I have done this multiple times and nothing as happened. I also went to a physical store only to be told to call customer service. How do I get this resolved?