Debitting money from the non agreed account

I have 6 year relationship with MTN, I have been their subscriber for the past 6 years and still have a contract with them until 2025 . The problem is they decide to debit from a different account, I gave them the account when I was trying to contract a new phone . The have been debiting from Capitec for the past 4 years and all of a sudden they debitted the wrong account . Now they say I owe them money for the past 2 months