Overcharged every month, how do I get this resolved?

I've spent total at least 6-7 hours sitting in a T-Mobile store trying to get refunded for overcharges on a cancelled line. I finally got ahold of someone who said they were going to refund me, I said "Great, thank you so much, this nightmare is over!," but I am SO DUMB and didn't ask for them to send me this in writing. I'm not sure it'd have even made a difference. It's 3 weeks later and no refund has shown in my account. T-Mobile seems dead set on scamming me and giving me the runaround. I was a customer for nearly 20 years and would have considered going back.