Mtn
Account paid up but still have balance

My account with MTN was paid up last year and have made the effords to contact them to cancell it, but to my suprise they handed me over because i owe them money, i have called to inquire about this issue but all they can say to me is "my account details is not on the system" i mean really if i owe you guys money how come my details does not appear on your system??? this is better get sorted cause am tired of getting sms's from the lawyers, and will not pay something i dont know about as the account was cancelled!!!