Frustrating customer services

I had a data contract which I cancelled in May 2018 and have still been billed for until this month. I have called your customer service a few times and am being sent from pillar to post with 3 or so employees promising to call me back and I have still as of yet heard nothing back since I called two weeks ago. Besides which every department requires a lengthy security check and I’m on the phone for hours every time I try follow up. No one could provide me with a reference number either. I have broken my leg so am unable to go into a store and am completely frustrated by your companies inefficiency. All I want is for them to go back into my call histories to hear that I cancelled the contract so I can be reimbursed for the debit order going off for 8 months unnecessarily. Please assist me.