Complaint about owing for old landline

Hi I was going to renwe my cell contract but was informed that I had outstanding amount on my old Telkom land line.i cancelled the line and never used it.i dispute that I owe so much as I sold my old place yet Telkom still charged me.i have a current contract and was willing to upgrade meaning more money for Telkom yet you want a miserly 230 old from me.if I cancel my cell contact you going to lose me as a customer forever. .