I was asked to call back but can't get through to talk to an agent

I received a phone call from Xfinity about missing equipment and the bill that supposedly was not pain in September of 2023 even though this whole time every month was paid and there was nothing on the account suggesting that something didn't get paid and when the account was closed there was actually -$100 that they were supposed to reimburse me that I never received.  It has been almost two years since September 2023 and I never received a claim or past due from them until now. I've been asked to call back. I have closed an account a few month ago with $0 balance and now all of a sudden I owe money from 2023 and I was also billed for equipment that I never purchased or rented from Xfinity. This is the third time they are trying to bill me for something on the closed account. I've talked to abut four agents now and everyone said that I won't hear from them again and everything is cleared and settled, and then I get another bill.