Unfair Charge

I took a month to month contract with Telkom last year. I have been utilising their services since then until I had to go to the field for 6 months as I work for the department of defence. My bank account had problems leading to Telkom not being able to debit my account, I could not fix my account as I was in the field. So I assumed it won’t be an issue since I took a month to month contract but to my surprise I’m being told I owe them an amount of R1800 for the months they couldn’t debit. Maybe I don’t understand what it means to go into a month to month contract.