Telkom is a **** and should not be used

I have an ongoing cellphone contract with Telkom in which they debit my account monthly for the contract amount. But they keep changing the cost of the data bundle linked to the contract making the debit order insufficient for the bill owned. But when they debit my bank they don't pay the amount owed from the previous month, so after a year they say I ow them like R20 due to the data bundle price changing. How is this correct, like if you debit my bank account I am paying my account, you can't say I ow money from previous months then, because I ow you for this month. Then they never contacted me about this but family members who aren't on the same account as me at Telkom did an adverse listing on me due to this.