Debt collection threats on CANCELLED ACCOUNT!!

Yesterday I started receiving emails and sms's that I have been handed over to some collection agency due to non-payment on my Cell C account! This morning I receive a letter from the collection agency stating that my credit score has been changed etc!!! This contract was CANCELLED in February 2021 (I have the sms's to prove this). I also spoke to Cell C accounts when it was cancelled to make sure that the account has been closed and asked if there was any outstanding cancellation fees etc, to which I was told NO SIR, THE NUMBER HAS BEEN MIGRATED TO PREPAID. Yesterday, without ANY calls or emails for the past 4 months, I start receiving letters and messages from collection agencies!!!!?? PATHETIC SERVICE AFTER 12 YEARS AS A CELL C CUSTOMER!!!!