Unfair service by Vodacom regarding contract cancellation

May 2018 I settled my contract by paying a sum of R5000.00 into Vodacom Nedbank account. Today 27 July 2020 I discovered and informed by ITC that I am owing a sum of R5500.00 to debt collectors (on behalf of Vodacom). In order for me to open an account I need to resolve this debt first. It is difficult to comprehend how a renowned company as Vodacom do not inform their clients that the contracts are not cancelled automatically after settling a contract bill in full. Furthermore I do find this unfair unaffordable and wish this not to happen to anyone else. To date I have not recieved any correspondance or notifications regarding this debt to Vodacom.