Incorrect Billing

I cancelled one contract as from the 31/3/2015 and put it on a pre-paid. I still have another active contract. On the 3/4/2015 I received an account for R1735.68 of which R1655.95 is for a 24 months handset instalment. I have contacted Vodacom on 4 occasions and no one can tell me what this amount is for. I told the last consultant that I was going to reverse my debit order which I have done And until someone contacts me I will not pay this account.