Billed after contract ended

My contract ended beginning july, my acc got debited on july 1st . I called to enquire and was informed that i should have called cancellations a month in advance to cancel. I didnt know that. Anyway i did that i got my account migrated to prepaid effective 2july. However today my account got debited again for R537. I went in to a store to check because the call centre was absolutely pathethic as they never even take my details to see what is the problem. The shop advised that my acc is cancelled so i shouldnt be billed. I would like a refund as my contract is completely paid up. Can someone who is willing to assist please call me . Kind regards