Contract cancelation dilemma

I am so furious with mtn I have 4 contracts with them and pay my accounts every month and has never skipped a payment. One of the contract is ending in 2025-04-11 and I went to their store to put in a cancelation notice and I was told I need to call 135 and I will be assisted, I then call and the lady by the name of nosiphiwe mlugwana told me that she can't assist me as it shows on the system that my account is in arrears and I have been handed over therefore she can't assist me I then told her I never missed payment and she referred me to the people who deals with accounts and the guy that I spoke with told me that their system says I owe I told him that I have proof of all my payments on the banking app and I have never missed payment. He said there is nothing he can do I must call next week to see if the issue has been resolved. I want to know why when I have to cancel my contract I am told I owe mtn when I have proof I have been paying? I want that contract cancelled because I don't owe anything and I have proof of payments