Poor systems, Bad service, Dont care about customers

Useless systems put me in debt with Vodacom Vodacom failed to debit my account for over 5 consercutive months, and I reported to them they kept saying they'll solve the matter and each month it didnt go through. I requested a account/reference so that I pay over the counter but they refused and said it not allowed to change method of payment. Each month we had the same issue and they kept trying to fix it, each month the installment due increased and they stopped the benefit of the contract. Then started sending letters of demand which came with the bank details which i have been requesting. Now i have to large overdue amount caused by their poor systems, even threaten with lawsuits. How does that work, is it right to not take your money and accumulate interest and then start demanding payment. This contract pretty much doesnt exist since Vodacom were in breach of contract to debit my account, I will not be liable for all these interests that have accumulated.