Cell C billing me for a cancelled Contract

I app**** for a 24 month cell phone contract in 2018. When I wanted to upgrade the contact in 2020 I was told that the account is on a business name and it is not possible to upgrade the account. I went to the Cell C Branch in Festival Mall because the sim card no longer worked and I wanted to upgrade the handset. Again the branch said that they can not help me because this is a business line. As Ive already had the contract for 24 months I told Cell C thank they must please cancel this contract

I was surprised when a collections agent from Cell C just called me to inform me that this account is in arrears. According to Cell C the last payment was June last year.

I fail to understand how they did not call me when the account was 30 days in arrears and why they waited 15 months to call me. I have no handset and the sim card doesnt work anymore. How can you bill me for something I cancelled and now using anymore.