Poor and inconsiderate service from Vodacom

vodacom account installment is debited every month from my standard bank account and this month they made an error and returned it. when they contacted me the money had already been used up. if arrears have been incurred As a result of vodacom not having debited the way they should as stated on the contract signed between vodacom and me( customer). Vodacom must debit like they do every month to avert this inconvenience. Now they have cut of my outbound calls because of their error. This is just plain poor customer service. I cant wait for this contract to end so I can end my relationship with them.