Uncanceled contract as per customer’s request

To say I'm disappointed would be an understatement, if I could give negative stars I definitely would. I cancelled my contract with Vodacom in July for two cellphones *** & ***) and I only realised in August that they only cancelled one phone***). They expect me to make payment for the remaining cellphone***) whereas it is their fault that it is not cancelled. My bill ranges up to R700 now while I cleared my account when I was cancelling. Every time I call they just keep transferring me from one department to the next. Right now their collection department keeps calling me and I have to explain this all the time. This is ruining my credit score.