Vodacom keeps on billing me after contract is cancelled for 2 years

On the 21 September 2016 I cancelled my contract with Vodacom in the form of getting a settlement quotation from them and paying the outstanding amount .I send the documentation thru and received my Ref nr EC-04AL-3KW56D. My number was released and my Vodacom number was converted to Cell C . Vodacom kept on sending me a bill for my account and I took it up with manager of the branch . On the 20th of May 2017 I received a mail to say everything is Sorted and a credit was put on my account of R 3237.43. Vodacom phoned me again from the collections department saying I owe them money for my account . This is pathetic and every time you phone Vodacom you get transferred from one person knowing less than the next.