contract cancelled but still mtn claim I owe money after so many months of cancellation of the contract

MTN claims I owe it money,made several calls and requested a recording as it takes about different invoice which was not told about,I even told the consultant that they must be transparent with clie ts when rendering service,MTN talks about invoice of 500R and the contract was told about was for 99R,1 gig at night 1 at night,I really don't know which invoice is talking about cos I even cancelled the contact and MTN stopped giving me data as the contract was cancelled few months down the line I receive sms from MTN claiming I owe it