Angry customer

Hi I took a cellphone contract with telkom and fully paid it and canceled, after contract cancelation Telkom comes back to say I owe them money my first problem is Telkom said they can't cancel contract while a contract is still owing, then how was my contract canceled?, second is why is the money that I supposedly owe not fixed I keep getting different amount from their agents. The third thing is Telkom has never sent me an invoice or tried to communicate with me regarding the money that I'm liable for I'm failing to understand how I owe them after the contract was canceled cause I didn't receive anything