Dispute of debt

i had a contract in 2011/2012 with Telkom eita(Kenilworth Branch) and paid monthly installment for 24 months without missing a payment, after 24 months they billed me as a result they claim i owe them 720.00. now im listed as a debtor and im stuck with outstanding amount they claim i owe, i have even sent them email requesting cancelation after 24months