Paying for the phone i did not recieve

Hi my name is Felicia, I have a contract with MTN on *** and *** and I took a tablet on one of the numbers and after a year MTN called me asking if I want to upgrade on my other number, and yes I upgraded but the following day I called MTN to cancel and they gave me the ref number but I don't have it. after 2 days I cancelled, the courier called me for delivery but I said to them I cancelled with MTN and they said they will return the phone to MTN, Last year October MTN deducted 570.00 instead of 380.00, I phoned MTN 3 Times and they kept on telling me that they will call me