Billing me for something i dont have

I cancelled my contract with cell c on the 23rd of may 2015 and sent back the phone that they sent me for upgrade with the sim as i was not happy with it at all. i have been without a phone and sim card since the 23rd of may and they took the debit order for June (which is fine) and they took another debit order for the full amount of the installment on the 1st of july, how the f*ck can they keep taking money for something that i havent had in my possession for more than a month. i had the new phone for one day then arranged for pickup and they came to collect it two days later, that same day i arranged for the cancellation i even received an e-mail that said the cancellation has been processed. THEN WHY DO YOU KEEP ON TAKING MONEY FROM MY ACCOUNT!!!