Unfairly Defaulted......

I made a dispute in August 2020 agaisnt MTN, they had blocked my line for a missed payment in april 2020, after i sent them proof of payment that i indeed made that payment they couldnt assist me, i went to a branch and after several calls and back and forth i asked that the contract be terminated, the termination...i was told there was no outstanding payments and then the contract was cancelled, today my credit report shows i have been listed with a Default, i have not received any emails, calls, sms to state i was owing anything since Aug last year....i need this default cleared, this is damaging to my credit record and utterly disgusting service which is why i cancelled the contract to begin with.....