Sprint Lease to T-Mobile Agreement

I just found out that I've been paying my lease for 18 months more than I should have. The representative informed me that they were supposed to make good on our lease when they merged even if it wasn't up. I'm trying to find my lease agreement and so far can't find anything other than previous bills dating back to Sep. 2022. According to my phone, my last reset was in May 2022, so it was either then or earlier that my lease began. I don't remember if the lease was for 18mo or 24mo. According to the rep though, they should've stopped charging me 18 months ago. At $25 a month that's $450! They only gave me credit for two months. So I've been paying them for a lease I had with Sprint. That doesn't sound legal to me and I'm going to contact corporate. Any other suggestions?