Device payments

My son had upgraded back in December of 2022 to a IPhone 14 when they came out. We should had pay off the device by now but apparently he still has a balance on his phone and 8 more installments payments that don't go down. I went in the Xfinity store by me and they stated that apparently my son was being billed the $35 per month for the 24 month and it started billing him in August of 2024. The chat person stated there was a technical issue and glitch. At no time did we receive notification that he wasn't billed for the device and they didn't send me a letter explaining what happened. This is not my problem it was a problem with Xfinity. We been waiting to upgrade and we according to them it shows in my statement he wasn't billed. I been with Xfinity for a long time and it would be a bad thing to leave Xfinity to another company after so many years and always paying on time. This is not right that I have to start the payments again just because of there error. I need someone to look into this before they lose a great customer.