Vodacom charging for the settled contract

I paid off Vodacom contract a month ago but today I woke up to an invoice that I am owing them R1725 , I don’t think I will ever have an contract with them ever again, they are a bunch of *******, and when I try to call them to resolve this I will been sent from pillar to post by their useless consultant. And now I will have to reverse the debit order which I am going to be charged by the bank.