DISGUSTING SERVIVE - MTN

I have a Data contract number o*** for the pat 2 years where I ha been paying an amount of R260 pm I sent the MTN customer service in July this year requesting not to renew my contract on expiry date . I received an email from the MTN retentions department on the 27 July that my contact would terminate on tn 18/8 2022 and the number would be converted to a prepaid number thereafter. I now receive an invoice for an exorbitant amount of R1167.55 for 40 gigs Data being issued from 16/08 to th 17/09/2022 due on the 1st October 2022 although the number has been placed on Prepaid. I've sent at least 20 email asking them to rectify this incorrect invoice but I am transferred from one department to another - DISGUSTING -DISGUSTING -DISGUSTING SERVICE and lack of understanding from such a corporate